How does this happen?

Kasen put his undies on my himself today. Usually he does good, not sure what happened today..... Ps... they are fully potty trained and rarely have accidents. Although.... Thomas was putting a diaper on him the other night, and Kasen REALLy didn't want to wear a diaper, and he peed all over Thomas. The next day he got really mad because he got in trouble and had to put his nose in the corner and he peed on the carpet, so i'm pretty sure he is using his new found freedom as a weapon against us. I hope he doesn't keep that up and pee just to get back at us. He got in big trouble both times.... so he better not do it again.