On the surface, it seems like a subconscious reaction after being frightened, but in fact, is this really the case?

The action of Steve Rogers to stand up and block Peggy Carter behind him did not involve other thoughts, it was indeed just a subconscious reaction. After all, Peggy Carter has a very important position in his heart, and the moment he subconsciously switches to the alert state, the actions he poses are not so exaggerated, they only enhance the defense.