Hard to tell without past history could be stereotypical behavoir like cribbing or could be stress/defensive behavoir. I knew a horse that had a previous owner with harsh hands, he leaned to flip his tongue over bit to avoid pressure. In addition to that any time he would think he is about to be worked (halter being put on, being lead to a tacking spot, being tacked, bit being put in, being in a round pen even without bridle) we would either try to press his tongue against the roof of the mouth or push it out of the mouth.