What triggers the amygdala (part of the brain response for the "fight or flight") during freezing moment is your encounter with something so sudden (vague). We reacted quicker, emotionally, because our amygdala receives the input first. The amygdala is responsible for emotional responses.
Our cortex is where the information is processed to make sense of things. Cortex can counter the effect of the amydala and help you calm down.
For example, you saw a black, long thing that looks slithering on the ground and freeze/jumped/screamed. You initially thought it was a snake. That is your amygdala responding. After 2 seconds or so, you immediate saw that it was just a hose. Your cortex is helping you make sense that that thing is not a snake, but rather a hose. Your body starts to cool down as a result.
