Because we are taught to assume that people who are threatened will naturally run or fight (or imagine that is what we would do if faced with a dire circumstance), it confuses us to hear that somebody might instead freeze, dissociate, or shut down.

A different level of brain activity is involved for each response: the mammalian fight-or-flight system, which is protective and keeps us from shutting down, and the reptilian brain, which produces the collapse response.