When I first took on Papillon he had a reputation for doing that. In fact, the loaner before me had put him in a hackamore but I didn't like it and I don't believe he was keen either, so he now back in a snaffle.
We've discussed this a lot at the yard and have come to the conclusion that he now does it purely from habit. I find that he plays about, sticking his tongue over the top and pulling silly faces for a few minutes then eventually settles down.
But he's not officially mine anymore as my loan ended the other day! :sad:
Sometimes young horses do that when they are getting used to the bit. Putting it in the correct spot in the mouth can help.
Tightening a nose band, and possibly using flash noseband too, will stop the horse from playing with his tongue and bit too much.
Some as said, will do this to evade rider's hands.
If nothing works, even changing to a similar, but different bit, there are bits for tongue tossers..
It can't hurt to have his teeth checked, though I wouldn't think that is causing the problems.
If the bridle is as far up as it goes and it's still too loose then you need a bridle that fits better. Can you post pictures of your horse in the bridle? Also, What kind of bit are you using on him?
I have experienced a tongue over the bit with two different horses, one was when the bridle was too loose and I somehow didn't notice right away. The other was with my mare in a single jointed snaffle. She would constantly play with the bit and flip her tongue over it. Once I tried a double jointed snaffle that habit stopped (along with some others). She just didn't find the single jointed comfortable. Each horse is unique in what they like in their mouth.
Can you share a picture of him with the bit in? If the bit is too big, this could also be a problem. Some horses don't like a single jointed snaffle and, due to the angle, they are relatively easy to get the tongue over unless you turn the horse into the joker by pulling the bit up really high.
Generally speaking, I agree with the thinking that this type of problem usually stems from one of two problems: 1) some form of discomfort for the horse, whether it be due to a rider with poor hands, an improperly fitting bit, a tooth problem, or 2) lack of mental stimulation, a bored horse will entertain themselves however they can and that often results in them playing with the bit excessively or otherwise "misbehaving".
You said you changed bits, can you show us exactly what kinds of bits you've used on him as well?