I believe our first instinct, as well as for most other animals is to first freeze, so that the predator won't see us. Remember we've got a lizard and a little dog in the head with us and all that. That's the default stuff the amygdala (or automatic survival doodad) will respond with. Adrenal stress training will help make new and better imprints in this part of our brains. Another reason why people experience "getting stuck" like that is if they begin percieving time in slow motion, and then it seems like they're not moving because last time they moved it happened a lot faster.
Back to training though. I remember a fight a fight I had once when all my training just vanished. The guy held me by the jacket collar. As I was training muay Thai at the time you'd think that wouldn't be the worst of positions to be in, but I hadn't trained enough. I reverted to just trying to punch the hell out of him, but his arms were longer than mine and when he flinched from the first few blows (and him holding my jacket collar) my arms were too short to reach his head.
Then his buddy comes around on my left and tries to punch me in the head. And this is where it gets wierd. I'm not consciously aware of the guy, but somehow managed to dodge or roll with the punch while kicking off to my side and plant my foot right in his ribs, sendin him on his ass off the sidewalk. His face looked something like this:
and so did mine. I looked over at my friend some distance away, but realized it was me.
That one kick was something I used to do as a kid in my room when I was bored, just kicking imaginary ninjas numbering in the hundreds day after day after day. So much for hobbies... Basically freestyle kata with some visualization I suppose. so long story short. It starts, I stand there going "Bduuuuuh" and then some odd bit off training meets it's criteria and it happens. I think training along those lines may have some merit.