You mean, bull rush an opponent an involved in a grapple? First of all, it wouldn't provoke an AoO from the opponent. Secondly, there's no rule adjudicating what would possibly happen or how successful. If all members of the grapple wanted to disengage from the grapple, then the bull rush would work as expected. If they didn't, then you need to decide what to do as it's not covered. I would probably required an opposed grapple check to beat everyone who didn't want to let go of the grapple prior to rolling the bull rush results. If any of it fails, then the bull rush fails.