Steering things back on course, if I may
I'd totally agree with the trapped solvents explanations, for any of the reasons given.
To illustrate, here's a little story:
We tend to prime all our repairs together, at the end of the day, to save wastage. When I was doing it, we hardly ever suffered shrinkage problems. Now the boss has begun to let our young lad do the priming, we are having regular problems (same product used).
He thinks that if he bombs 3 thick coats of 2k primer on the big repairs, with minimal flash times, he can go home early (and it'll 'hide' his poor filler work). The problem starts the day after. Upon hitting the primer with a DA, you can smell the solvents bursting out! I've even had it so bad, that the primer has popped right through the base and clear during baking!
Besides re-educating him, which is difficult as he is the boss's son, I've found one way around it is to block or DA the primer down early in the day, and leave it to sit as long as possible before final sanding. This seems to give the solvents time to escape, after 'breaking the surface', and shrinkage problems have been reduced.
Clearly, as stated above, the better way would be to take more time applying the primer, and allow plenty
of flash time between coats, but maybe my method of 'breaking the surface' could help a little also.