Keeps taking his diaper off! Why and how do I make him stop??

The other day, my 2.5 year old took his diaper off!! I had him in timeout in the playpen, then when I went to get him out, his diaper was off and he had peed all over.

Today, I put him down for a nap and he kept saying "no diaper" and taking it off again. He just had a bath minutes beforehand and attempted the potty but never peed or pooped. (He only peed in the potty once like a month ago). He will sit on the potty sometimes but won't go. I'm not pushing it at all.

Why is he doing this? And how do I make him stop taking his diaper off??