I had a pony who did this and when he got his tongue over it would cut his mouth up - it was horrible. It was a nervous reaction in his case.
There are a few things you can do.
We tried a rubber 'tongue layer' A bit of rubber that looks like a human tongue that you strap on to your bit so it lays up the tongue. Worked well but he chewed trhoguh the rubber eventually!
You can also buy bits that have an inbuilt, metal tongue layer - most often used on racehorses. They are quite $$$ so we didn't buy one.
Another option (And what we finally settled on) is a bit with a port adjusted fairly high in the mouth. Again - the port blocks the tongue from coming over the bit and also gives them some tongue relief which is often an issue with these horses.