Trapping solvents that could cause the swelling/shrinking can be done many different ways. Too slow a reducer, too heavy a coat, too little flash, not atomizing properly, or even too fast a reducer with too heavy a coat! It can be a combination of many. It can also be that too much was asked of the urethane primer like filling some of those 80 grit scratches that weren't completely sanded out with the 180. There are many different factors that could be at play here.