Why do some people freeze up when scared?

It's a natural reaction, much like fear or anger. Some people are incapable of moving when they get scared or if they find themselves in a situation where they are overwhelmed. Sort of like when a deer is crossing the road and suddenly freezes when a car or truck comes up on them; most people think that it's the light that scares them when in actuality it's the situation - they are terrified that they are going to get hit but at the same time can't move out of the path of danger.

It was probably because the muscles tensed up and get quite stressed. Or the body produces chemicals from these reactions. It's just a tensed feeling.

that's the first reaction i think that come to any other human, then your brain will process the situation and the next thing happened would be your reaction with your decision, might be to fight or run and ask help. ^_^