Steve takes a couple of steps back, too, because Zerxus is on fire (well, not on fire but surrounded by it) and that is a little alarming, but more importantly nothing he wants to bask in. It's an odd show of good sense for a man who didn't react much when Zerxus was angry in bed and letting flames touch Steve.

Or maybe it's actually mostly about letting Zerxus have his moment to do something with his anger.

"All right. So why are you this mad about it?" At least he's had some time to figure that out, and maybe some of why he's relieved is that the 'deal' Zerxus made for him is... still decent and serves Zerxus.