Mr Moynihan, 49, is widely credited with rescuing the 54-year-old consultancy from a slow bureaucratic death by slimming it down and getting it to focus on its strengths. Yesterday the firm said his departure "was precipitated by problems in PA's discussions with the Butten Trust (the firm's majority shareholder) about reforming and modernising PA's governance structure - changes to which he was personally committed, in the long-term interests of PA".
Mr Moynihan, noted for his self-confidence and direct manner, said: "Though I was not planning to leave at this time, I know that the firm is currently very highly rated in the marketplace and that there is a broad base of ability which will ensure continued success for PA."
It is understood that Mr Moynihan was unhappy with the way the Butten Trust, set up by PA's founder Ernest Butten, allowed former members of staff a big say in the running of PA and apportioned them a sizeable slice of its profits.
Mr Moynihan does not have any immediate plans, other than to help PA find a successor. The firm is talking to a head-hunters, and wants to interview external as well as internal candidates.
Just as Andersen Consulting has found itself in the news over its inability to agree on a new senior partner this year, so PA looks in need of good advice on how to manage itself.
Here's another surprise resignation - Andrew Cummins has stepped down as director of strategy at Inchcape, the motor dealing combine, "in order to pursue some more personal business interests".
Mr Cummins has been on the Inchcape board for five years and now wants to take time to think about his future, the company says.
A spokesman emphatically denies that Mr Cummins was asked to go. "We've gone through quite a lot of changes, and we are concentrating on larger businesses. He felt it was time for him to do his own thing." So will Mr Cummins get a pay-off?
"None whatsoever. It was his decision to go."
And here's another one! It must be the heat. Jack Rowell's surprise resignation as manager of the England rugby union team this week may have dismayed or delighted rugby fans, according to taste. But it certainly leaves the former chairman of Dalgety time to devote to his business interests.
It is these very interests that Big Jack's opponents in the rugby establishment have attacked him for, accusing him of being a mere part-time manager.
But I suspect the 6 foot six former second row, who coached Bath to an unprecedented nine Pilkington cup titles, will be laughing all the way to the bank. His highest-profile business job was probably at Dalgety, where he was managing director of Golden Wonder crisps. He left three years ago to concentrate more on rugby, ironically enough, and Dalgety then sold the division in a management buyout.
Last year he was made a director of Oliver Ashworth Group, a distributor of pipelines to industry, while this year he became chairman of Celsis International, a Cambridge-based hi-tech company.
Jack's first real business job was back in 1982 when he headed up Lucas Ingredients, a company which put the bread crumbs on food. There's a pun in there somewhere but I'm far too hot and bothered to make it.
Yes, its another one. David Clementi has resigned his non-executive directorship at Thames Water. This was entirely to be expected, however, since the former boss of Dresdner Kleinwort Benson has to concentrate on his new job as deputy governor of the Bank of England.
Thames Water's chairman, Sir Robert Clarke, expressed his "warmest congratulations" yesterday on Mr Clementi's appointment to Threadneedle Street, and said he realised the latter would have to leave the board.
And finally, Rugby Group has appointed Philip Bruce as chief executive of Rugby Cement, its UK cement business.
He will succeed David McAteer, who is due to retire in 18 months after a career in cement stretching back to a time when The Beatles were an exciting new pop combo.