Haha, well, in humans lots of things can trigger a vasovagal reflex, such as stress, trauma, having a medical procedure that really freaks you out... I have a terrible needle phobia.
Last time I had my shots at the doctor's (sometime in college), I was doing fine, the nurse withdrew the needle, I stood up... aaaaand sat right back down and almost passed out. It wasn't fun! At least I got free animal crackers and OJ. My mom,who is also a nurse (yeah, my mommy was with me, what of it?!) laughed at me and told me what it was.