I think they're fighting. If they breed, they'll lay eggs on some surface in the tank (usually something vertical, like a flat stone or something) and guard them. This just sounds like one of them has made the other one mad!
my breeding pair of oscars argue all the time...they also breed all the time. They fight to test their prospective mates overall fitness. Unless one fish is cowering all the time then you most likely have fish that are getting ready to breed. If you want to promote it put a clay plate at the bottom of the tank and watch 'em cover it eggs. In fact mine just did the it early this morning.