I believe the expression derives from early RAF (or even RFC) jargon for jumping out of an aircraft (with a parachute, of course ;-)) - which suggests that in order to 'bail out' of a meeting, you have to be there to start with: so "leaving quickly" is a good way of summing it up.
Of course, a pilot doesn't jump out of a plane unless there's something wrong with it. So someone might bail out of a meeting if it was obvious that it was getting nowhere and they wanted to get on with their work.
However, I've heard the expression used in the last 20 or 30 years (since users have become less aware of aerial combat) to mean the same as 'opt out'/'chicken out'/'decide against attending'. This is a minority usage at present, but I think that's the way the meaning is developing.
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