Wow why so many comments that don't understand simple accidents like this. Its the reason why parents tell their children not to sit on ledges and yet so many people still do it, without a backrest its really easy to lose your balance and fall over, especially if someone pushes you when you're not ready for it.
This was a pretty realistic manner for an accident to happen, things like this only seem that foolish in hindsight but I bet pretty much everyone in this chat puts themselves in situations where accidents can happen on a daily basis.