So it looks like incoming cabins from EPCOT into the rivera station collided with cabins already in the station. Cabins detach from the line and are motored through the station on wheels. So I guess somehow that movement was stopped in the station.. but the EPCOT line kept feeding them into the station. Or maybe somehow a stop happened in the magic window where a cabin was coming off the line and it didn't stop with the rest of the action?
Will be interesting to hear about it all when someone gets the real dirt.
EDIT: Looking again at videos.. I think it's the other way around. Cabins LEAVING rivera to epcot seemed to be the impact. So maybe what happened is the blue cabin failed to transition to the live line correctly.. and then the yellow cabins in the station fed into it.
The good news is, if that is the case, the impacts should have been at station speed.. not coming in at 11mph down to nothing.