There's a few reasons: mostly not understanding on some people's parts.

AST players like the one you encountered are fairly regularly around, only it seems to stem that whatever stance they are will be the exact same one that their Co-healer has if it's a SCH/WHM as if to show "look, i'm better than they are at their job".

NOT ALL AST players are like this, quite a few that I've run into are like this though and they're either a) 'over geared' (aka The Raider) or b)snarky and feel the need to just show off an e-peen for whatever reason.

I've come across many an AST that I've worked well with in my runs as a WHM. Where things just click and not even having any Discord contact, I can almost guess what they're going to do and I'm pleasantly shocked when my gut is right. However, far more often, I come across AST like the one you just ended up with. They want to 'fight' on terms that my WHM does, even though...SHIELDS WOULD BE REALLY NICE.