I think the primitive risk assessment part of your brain kick in too.
You KNOW it can't be real, but there is the little tiny part of your ancient mammalian brain that REMEMBERS those things and say "Yea... but what if you're wrong and it IS real?"
Yora I think the term suspension of disbelieve could be used to describe a situation where you logically know something is impossible but in the moment you believe it anyways. No I don't thing that was reflex, yes the first couple seconds but after that the situation has been assessed. I've seen when these pranks result in just reflex and the person runs for a moment and then stops and laughs. This is not that.