Explain To Me What Just Happened Here?
 

This doesn't seem right to me so I figure I would throw this out there...

I was watching my brother play and he was using his level 10 Iksar Monk in Cabilis. He was fighting a large Scorpion and had the Scorpion down to 10% health. Suddenly, the Scorpion starts attacking with multiple hits in a row and the attacks turned off his attack.

I don't have the actual logs but he read me the screen and it went something such as:

You miss.
Scorpion hits you for 26 points of damage.
Scorpion stuns you.
Auto-Attack off.
Scorpion hits you for 34 points of damage.
Scorpion hits you for 25 points of damage.
Scorpion hits you for 18 points of damage.
Scorpion hits you for 15 points of damage.
You Die.

I know a large Scorpion is classified as a Warrior and get that berserk attack when they are low on HP but for the Monks attack to turn off when stunned and then to get destroyed by four shots in a row? I know the character is only level 10 but it's more like a WTF just happened moment that I'm talking about...

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!

My brother has been playing long enough and has enough characters to know not to turn auto-attack off when he gets stunned.

Well the mob didn't turn it off for him, so his finger must have slipped or he got confused as Cecily said.

What you are claiming (that the scorpion stun or something the scorpion did turned it off) is impossible.

Truth, sorry to say.

Bumping his duck hotkey by mistake would also have turned off auto-attack (and prevented it from being turned back on). As the others said, getting stunned does not turn off auto-attack. I believe being in duck mode also counts as sitting (which causes a mob to auto-hit), though I could be mistaken there. But if that's the case, it could also explain why he got facemelted at the end.

Mob's melee push also could have driven him into a terrain obstacle that forced him to auto-duck.

Can almost guarantee this is what happened, especially if he plays in first person mode it would be hard to notice that your character was ducking. The default key to duck is X which is pretty easy to accidentally hit with your hand in the WASD position.

Did the creature push his character into a spot that made him duck ? Creatures can push players like we can to them

Lag spike?

most likely explanation right here