Usually it is intimidation or a call to play with him (if he's a young horse).
Either way, you cannot tolerate this behavior because then it will become an habit and if you support that he won't understand that it's forbidden. If he takes this habit, then he will bite you but in an angry way whenever he's angry (when you tighten the girth).
When he does it, just say "no" without aggressiveness and give a little hit on his nose to make him understand he can't do it.
I hope it will help you and he will stop this behavior :)