Explain To Me What Just Happened Here?

[Cecily]

Class benefits don't necessarily apply to mobs. They (thankfully) don't go berserker at low HP. There's other things they might do at the high end, but that's not gonna be relevant for you till way later.

What happened there is a mob proced a stun on your brother. He couldn't attack back because of it, got confused, and turned his auto attack off. Then the scorpion ate his face. Hope this helps!