Hello N., I would bet that there is something going on at preschool or daycare that is causing this regression. If there is, he wouldn't do it at those places, it would happen at home where he is more comfortable and relaxed, less tensed up. It could be something as simple as not being able to go potty without permission at preschool, causing him to hold it, then at night, he jsut can't hold it any longer. I would recommend talking to the teacher and asking to help in the preschool classroom one day very soon, so that you can see the structure and make sure that there aren't any kids picking on him. Also talk to the daycare and ask if there are any new routines or any new kids that he might be having conflicts with, but I would bet that it is the stress of going to preschool.
Even if he loves it, it is still stressful, just remember your first day of college. Put the pullups back on for at home. Volunteer in the class. Start reminding again. Make sure that he is getting enough sleep, might need just that little bit more. Please don't punish, I am sure that he is just as upset, and probably doesn't know why it is happening.