There can be multiple reasons for horses putting their tongue over the bit - its something you need to address as it can turn into a dangerous issue, you have very little control once the tongue is over the bit, and you can also cause great damage to the mouth if you try to pull back on the underside of the tongue.
Check that the bit is not hanging too low in the mouth. Discomfort with the type of bit is another factor, if you are using a single jointed bit, try a double jointed (french snaffle for example). Crookedness caused by discomfort elsewhere in the body, a horse may tilt the poll slightly to one side and bring the tongue over the bit showing discomfort and pain in other others. Dental issues. Crookedness and other issues caused by poor riding, if you are crooked, pulling on one rein, pulling back on both reins, jabbing the horse accidentally in the mouth etc. it is understandable that the horse may put its tongue over the bit. Evasion of contact in the bridle.
Address the root of the problem. Some will put a drop or flash noseband on strapped tight to clamp the jaw shut so the tongue physically cannot go over the bit. Please don't do this!