There's footage of Biden earlier where he's not wearing a helmet riding. This means someone gave him the helmet to wear, which probably means that security was already worried he would fall. Biden rode right up to the group of people where he fell, others rode around the crowd, I think he saw the little girl and wanted to interact with her before he fell, because he is a pervert pedophile. Interacting with the crowd was a staged event, you can tell because of all the cameras and people with lanyards (likely reporters). It seems possible that the little girl was an actor, that's how this clown show works.