system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does our body sometimes moves or shakes when We remember a cringy situation from the past?
gpt
If the situation triggers a flight or flight response, the body's first impulse is movement. 

A loud noise behind them will cause most people to start. This is your brain telling your body to fire up its muscles and get ready for whatever is about to happen.

Remembering a sufficiently unpleasant situation could trigger a similar flight or fight response, firing off muscles for the hardcoded classic danger response that nearly all mammals share.