Sometimes it's envy. Sometimes it's fear of a talented young rival. Sometimes it's downright prejudice.
Harassment at work has many causes. But whatever the motive, the result can be more than just a bad day at the office. Careers can be stunted and personal lives turned upside down.
Tim Field, founder of Success Unlimited, an organisation that offers support to victims of workplace bullying, estimates that 95 per cent of cases involve someone in a position of power making life difficult for someone lower down the hierarchy. And the bad news for teachers is that they top the list of victimised professionals.
Since Mr Field established his helpline three years ago, more than 4,000 of the 20,000 calls he's received have come from teachers. "They are particularly vulnerable," he says. "Most are committed to their job and won't do anything that harms the children. So they are more likely to put up with unacceptable behaviour. This gives the power-abuser the edge."
And in many cases this abuse of power is closely linked to sexual misconduct. This is more than a pinched bottom behind the filing cabinet, or a lewd remark by the photocopier. It is typically a long-term campaign of attrition.
"The dominant professional will instigate affairs, often with vulnerable employees," says Mr Field. "It bolsters the feeling of power. But if, say, the other person ends the affair, that power is undermined. Then there's bound to be trouble."
This is precisely what Margaret Bentley (not her real name), a middle school teacher from north-east England, discovered to her cost. Having ended her year-long affair with the married headteacher of her school, Ms Bentley is now unemployed and disillusioned.
Her case is a warning that the measures in place to deal with workplace harassment can be inadequate.
"I was very naive," she says. "When things turned nasty, he warned me that I'd lose everything. He was right."
Although with hindsight she believes she was "primed" for a relationship, at the time she willingly began the affair. As a single mother she felt vulnerable and was flattered by the attention of an authority figure 20 years her senior. But when she chose to end the affair, the harassment began.
"He made life difficult for me. He would ignore me, but he would also go out of his way to obstruct me. He denied me teaching materials and refused requests for assistance. It was impossible for me to do my job properly."
Things got serious when the head warned Ms Bentley that she would soon be "packing her bags". A few weeks later - in the staff toilet - he told her she was being placed on a charge of professional misconduct. She was accused of publicly criticising the school, and he claimed he had a parent willing to give evidence.
Tim Field says this escalation of events from obstruction to intimidation is typical. He explains how petty and seemingly trivial attacks are used to weaken the resolve of the victim as the prelude to a final "elimination".
"Undermining someone's capacity to function effectively in their job is the classic strategy. A skilled bully will hamper someone to the point where he or she can be subjected to a competency procedure."
Unwilling to be "eliminated", Margaret Bentley opted to fight back, a decision she now regrets. She feels more let down by the system than she ever did by her erstwhile lover.
An internal grievance procedure was set in motion at her request. The case was aired before the school's governors, where she believes she won some support.
But her hopes were quashed when, she says, the head used his position to force through decisions in his favour. The chair of governors then resigned, objecting to the pressure put on him to remove Ms Bentley. He was replaced by someone Ms Bentley believes was more willing to do the head's bidding.
Tim Field recognises this manipulation of governors. Often, he says, the head's close allies are drafted in to key positions. At other times the governors are simply too weak to take action against a strong head.
"Governors are essentially volunteers," says Mr Field. "They tend to be well-meaning people, and when everything in a school is running smoothly, the governor system works well. But they aren't trained to cope with conflicts of this nature."
The local education authority tried to arbitrate, but its impartiality was questioned after a disclosure that it had already guaranteed financial and legal support to the head. It also failed to implement its own written harassment procedure, under which the head should have been suspended until the matter was resolved.
Ms Bentley was left with no choice but to go to tribunal. Her case collapsed on a technicality and she was forced to accept a settlement offered by the head's team. Elements of this settlement have still to be honoured, including finding her a suitable alternative post.
She believes she has failed to clear her name and says that rumours circulated by the head - including a suggestion of mental illness - have made it difficult for her to rebuild her life.
"I always had faith in the system," she says. "I believed that if I needed help it would be there. But that wasn't the case. The tribunal was a farce. It was run by people with no qualifications or authority to judge."
Andy Ellis, of Ruskin College, Oxford, sits on employment tribunals and has some sympathy for Ms Bentley. He warns that unless you have a single strong incident about which to complain, convincing a tribunal of long-term harassment is difficult.
"In the United States there is an offence of 'creating a hostile work environment'," he says. "We need something like that here. At the moment matters such as racial discrimination are taken seriously. If someone's just waging a personal vendetta, it is much more difficult."
Surely, then, it is times such as this that make the annual union subscription worth every penny?
Tim Field says:"Teaching unions vary enormously in the support they give to members being harassed at work. If your union doesn't support you, you are banging your head against a brick wall."
Margaret Bentley's union advised her to "be more assertive" and resolve the matter internally. She was financially cornered without its support and could have faced a bill of more than pound;20,000; tribunal costs can be as much as pound;1,200 a day.
At that kind of price, the unions are forced to choose their cases carefully. In a case such as Margaret Bentley's, it comes down to one person's word against another.
A spokesperson for the National Union of Teachers says complainants sometimes expect too much from the harassment procedure. "If someone fails to get the settlement they're looking for, they lash out. People blame their union, or the tribunal, but it's not always easy to pinpoint faults."
Ms Bentley's is not an untypical case, says Tim Field. "When teachers start to tell me their story, I more or less know the ending," he says. "The similarity between cases is astounding."
His advice to those who find themselves the subject of personal harassment is practical if dispiriting - get out and find a new job. "The odds are stacked against you. It may feel like running away, but the alternative is almost always worse. Once legal proceedings start, the chances are that nobody will win. If you go down that road, be prepared to write off a decade of your life."
What to do if you think you're being harassed
* Keep a diary of incidents, no matter how trivial they may seem. Tribunals are unlikely to be swayed by vague recollections.
* If there are witnesses to the harassment, get their co-operation immediately. The further things go the more chance there is of witnesses being afraid to get involved.
* Most abusers of power are serial offenders. Find out if others have been targeted.
* Contact your union as early as possible.
* Insist on a third-party presence at any meetings with governors or local authority officials.
* If it does come to legal proceedings, choose an experienced solicitor. Employment law is complex.