Have the teeth checked. If that does not help right away then he's just sloppy.
If his teeth get floated and it helps for a few months and then he starts doing it again its probably because he had really sharp teeth before and they just got sharper wuicker than normal. For some reason horses that have really overdue teeth should be checked again in 3 months to make sure they did not create new sharp points because the pain was taken away and they started chewing differently. After that 3 month check they should be set to go for another 6 months. Horses between the ages of 5 and 20(depending on hardness of teeth) should be checked every 6 months.
My sister's pony does the same thing. (its called quidding)
He gets his teeth done routinely every 6 months and maintains his teeth fairly well between floating (not many sharp edges when he does get done)
His seems to be more of a nervous habit as opposed to dental health issues. He does it a lot less if not bothered, but if anyone is near his bucket or stall as he eats he tends to drop/fling a lot more grain.
So if its not a mouth problem it can be due to other reasons. I just like to be careful about his feed area to be sure that when he goes to pick up the grain he's dropped that there isn't any dirt or other material that he will ingest along with it.