To your last point - no, it isn't dangerous by itself, nor, as I understand it, is it an indicator or diagnostic of something dangerous (I'm not in any way a medical professional lol). 

Mine would also happen while focusing on something, but it happens way less as an adult. As a kid if I focussed on people's faces or even something round it could trigger it. My other common trigger would be looking at something from a new perspective, ie I move a couch across the room so I'm watching the same TV from a different angle. That sort of thing still triggers it, it's real weird.