Bruce doesn't even get to enjoy the whiplash run of Tony's expressions with the majority of his attention on Zero until the brick destroys one of the suits.

Well, shit. There goes my ride home.

Which promptly becomes the least of their worries when Tony goes for a fall.

"FRIDAY!"

Good thing there are two suits.

Bruce has barely enough time to shout for FRIDAY - and he's going to count on her to prioritize Tony's safety - before he's backing away from Zero's terrifyingly fast advance and getting some airtime of his own.

Anger might be suppressed, most of the fear tied to it might be suppressed, but when it comes to survival, that particular fear is tied to instinct too fundamental to life to be suppressed, and Bruce grabs onto it like the literal lifeline that it is.

Fear - he knows just how much it's going to hurt to hit the ground.
Fear - Tony's falling now, too, and all he has just that moment is hope that FRIDAY will get to him before the brick destroys that suit, too.
Fear - that goddammit again he gets close to someone and it leads to them being hurt or maybe even killed.
Anger - that this is his life.
Rage - that this is his life.

And there's that trigger, right in his grasp. He reaches for it, and starts to change.