My older son was doing this for a while when he was about 3 too. I think it was just the novelty of it all. He would go more often when we were out in public, I remember once we were at a playgroup for an hour, and he went pee 3 times! Whenever he would see a public bathroom, he automatically had to go!
He eventually began to hold it for longer, now I think he can go 2 or 3 hours between pees.