I'm not really sure how to be more specific, since it seems to be so broadly affecting people, and I rarely see what the cause is. Can't seem to reproduce it with NPCs.
I just saw the "Evil Eye" ability on Darkling Assassins trigger it. Even when the PC was no longer under the effect of the ability, they continue to blow a non-existent guard horn. I used the DM heal to completely strip them of buffs and debuffs, and they still blew a horn twice afterwards.
So far it seems pretty certain that any Fear auras and Fear gaze effects seem to set it off, though I believe I saw it happen once with a Confusion spell in Elketh's tower.