Don't worry, this is perfectly normal behaviour for pack animals, which includes rabits.
Anywhere where there is more than one male, one of the males will establish superiority, and occasionally remind the other male(s) who is boss. It's nothing to worry about - as you say they co-exist quite happily the rest of the time, so the occasional spat is part of living together. If your 'underdog' rabbit doesn;t appear to be suffering, or bullied excessively, just let them get on with it.
Make sure you keep an eye open when your 'top' buck becomes ill, or just old - the tables may turn, which again is nature, but he may be past defending himself by then - could be time to separate them - but that's a while off yet.