@chenando1 remember we do not see the full animation and illustration of the action on top sometimes they can be inaccurate with the original portrayal. he could have attempted to shake his hand off in aggravation using some force and the sudden shock of this action causing him to lean back with both of these he could be knocked over, also if it was portrayed wrong as he swung his hand to reject the hand he could have accidentally hit his chest and pushed him off by accident. there are several ways it could happen but we only see a single panel of this reaction.
@FunZed
Most of the accidents are stupid. Look at the real world, drivers falling to the river because they were playing Pokemon Go, some woman got electrocuted because she was trying to pull out the broken pin from power plug from socket and she was doing it with metal pincers, or from my family my 12 years old and 10 years old cousins, many years ago, were racing towards home and on stairs, one of them grabed the other and fell down. He had broken skull and towards this day he has problems with memory. So yeah accidents are mostly created by stupid actions. (Ugh, my english grammar sucks).