i think gravity created a force strong enough to push him down.but it also depends how slow or fast the bus is going
Explanation:
gravity as we all now it is pretty strong so if the bus is going fast then the person standing up will fall but if the bus is going pretty slow then it'll just nudge him.
Their function is to produce sound by allowing the free edges of the folds to vibrate against one another and also to act as the laryngeal sphincter when they are closed.