I'm sure this is some kind of defensive behavior, I think they hiss to try to scare you off before playing dead? I'll be the first to admit that the title is not meant to be a scientifically accurate description of his emotional state.
Imagine you just chill in the grass and this giant grabs you. Their enormous hand wrapped around your whole body they lift you up, stare at you. Are they going to kill you? You never felt this much fear. So you give it your all and scream from the top of your lungs. You show a level of aggressiveness even you weren't aware of inhabiting. You never felt this much rage. You are going to show this skyscraper of flesh who's boss!
But the giant is unimpressed. They hold this big, black, rectangular object right in front of your face. You don't recognize it. A weapon? Is this it? The giant lightly taps on the object. Your life flashes before your eyes. But then the giant puts the other-wordly object away. Your eyes meet again as the giants' mouth starts opening. That's it, they are going to eat you! You close your eyes hoping for a quick death.