I get frozen like this when I'm stressed. Repeatedly and at random.
By "at random" I mean that even though freezing is, for me, a completely non-random reaction to stressful thoughts, the thoughts themselves come at random, in an invasive way, and they very often have nothing to do with my immediate stressors. So I call it random.
I know things are bad when I'm freezing. When this happens to me, it's time to sit down and evaluate my environment and commitments, because something's gotta give before I break.