I have a similar symptom, but it's usually because the trigger is so extreme that my mind "shuts off" in order to protect myself I suppose. I don't recall "freezing" without a trigger though and I've fortunately never done it in public. I'm kind of afraid of it happening while driving or something. Fortunately, I can avoid the worst of the triggers.

For me, this doesn't happen if my body is too tired. My body is always too tired. But that doesn't mean that that isn't the reason for you freezing. Are you aware of a specific trigger that makes you feel paralyzed?