This happens to me sometimes.
If I am in a stressful situation or probably even any situation where the nod would be noticed by someone I would not want to notice I "scare" myself. Maybe scared isn't enough to describe it properly. Several times I have come back from the nod to a loud BANG which is me hitting the keyboard with something, my hand I think, and I am a little confused for a few milliseconds, and then agitated because I shouldn't have been nodding by that point.
When there is not any stress and I am alone or with cool people I enjoy the nod, it is encouraged, played with like a toy, and I do everything to manipulate it sometimes. Less stress always means enjoyable nods,higher stress for me are scary, agitating, frustrating, and difficult to stop once you hit that zone. I can get them the day after dosing if I sit still and concentrate on one thing while allowing my mind to wonder. No, I'm not falling asleep hah, I know the difference. 15 - 17, 18 hours after my dose I can get strong nods that put me out. I have tried hitting this point with caffeine, and a couple of other things which are stronger and that is not the solution, at least those things were not.
On Topic: I have scared myself "waking up" / "returning" from a nod from physically hitting or bumping things and making loud noises. But I do not think I am more prone to being scared. I am more prone to handle situations and get things cleared up one way or another though, heh.
Opioids do have a tendency to make people irritable if you interrupt their daydreaming. I think this is well established.