We can agree that neither of us know for certain what happened.
As for the bus route, I think that has more to do with heading off a lawsuit than anything else.
Consider this picture of a typical emergency door latch:
That red lever is what opens the door, it must be pushed either to the right or left (not sure which), but what I am wondering is whether in the struggle, one or the other of them could have hit that lever with their arm causing the door to open. If they were leaning against it, they both would have gone out.
If that were the case, it would have been entirely an accident, with no forethought, premeditation, or criminal intent. Just kids doing stupid stuff, as kids are wont to do.