While not tripping, there are various things that could work as they do allow you to move opponents in various ways. It might however come down to DM variance.
Drag: Grabbing the target and hauling him out of the saddle by main force(straight backward)
Bull-Rush: May need a jump check(Flying tackle)
Reposition: Drag an opponent around(with the exception that it must remain remain within reach)
There is also the feat Pushing Assault(APG)(you need power attack, STR 15, BAB 1 and a two handed weapon as pre-reqs) which lets you push an opponent backward.
There are also the monster abilities Pull and Push that do much the same as Drag/Bull-Rush(with the limit of it must be a smaller creature than the monster)