Son is going in his pants!!

My son has been fully potty trained for over a year now. This past week he has started pooping and peeing in his pants!! It's not all the time but I can't figure out why he's doing it at all. He doesn't seem sick in anyway and doesn't seem to have changed in any other way. Any ideas as to why he might be doing this? When I ask him he just says that he's being silly. He doesn't go in his pants while at preschool, just at home.