Yeah that is pretty gutless. And it's not like it was some rage induced deal that you could point to for him doing it, either.
If you're going to allow fighting, this stuff will happen. Abby stepped up for fisticuffs and until he's down, he's gonna get punched. I don't think Brouwer had to go so hard, either, that's true, but... no way for him to know if Abby isn't winding up for another swing after he regains balance.
But then again I'm one of those hockey fans that thinks the modern NHL game wouldn't change literally 1 inch if you removed fighting. This fight is a prime example. Big pile of why? It's two guys way behind the play dusting it up because they got tangled. And because it's Abby and not Nyquist getting tangled, gotta fight it out!