Get his teeth and palate checked. Let the dentist know your problem - Ask if its the bit interfering with the palate incorrectly or if his teeth need some care. If he tells you theres nothing wrong and your horse keeps doing it, get him a figure-8 noseband. Most likely he's just bored if there's nothing wrong with his teeth.
It can be many things. Stress or pain are common ones. That and if the horse isn't used to a bit they can chew. But my gelding chews on his bit and we believe it's because he enjoys playing with it. It's not a bad thing to chew on the bit if it's because of boredom. Chewing makes my horse salivate, which is good. He has a very soft subtle mouth and it helps.
But if you don't know why your horse is doing it get your horse checked out, see if it is pain. Get his teeth done etc.
The vet checked his teeth about a month ago- his wolf teeth are out and there is nothing wrong with his teeth/mouth. I also think its because he just likes to play with it.
Well he might just like playing with it.
You changed his bit recently didn't you? It was a non-jointed bit wasn't it? And you switched to an egg butt snaffle? It might be because he's just found he can play with it and move it. Haha. I wouldn't be too worried, unless you can see it's causing him pain/trouble. :)