There's someone lunging at him, too fast too large too close too close tooclosetooclose.

He doesn't see who it is, doesn't hear the apologies, doesn't stop to think, he just lashes out with the metal arm, panting with fear and reaction to the flashback as much as the present. He's not in a brutally utilitarian cabin on a prison ship, he's in a bunker somewhere in Siberia, and the handler isn't there, didn't finish the sequence, just left him half-primed and hurting and terrified. And someone lunged at him.

Unless Steve is very quick on his feet, he's getting launched backwards into the wall by the door, from a metal forearm slammed into his ribs.