I think it’s actually reasonable to expect fights after big clean hits. Clean hits can still result in injuries so you still want to deter it. Often the fight initiator might be fringe guy looking to get noticed and make an impression on his teammates. Again, reasonable to expect someone to do whatever they can to stay in the lineup. Their career earnings are on the line.
A lot of the time, though, the player that got hit is clearly fine but his teammate still steps in to fight. Often that player didn’t even see the hit, just the aftermath. I recall an example last season where they even fought the wrong guy.