I think that happens because in multiplayer replays the car's movement is not stored as precise as in singleplayer.
However the movement of objects is not stored in the replay, instead objects physics are calculated anew every time the replay runs. So when a car's movement is just slightly
different than in actual race it will hit the object at different position/speed and the object will then move different, too.
Sometimes if you rewind the replay you get different outcomes for the objects. (also sometimes one is the original behaviour)
By the way thanks for the videos, my LFS computer is still broken, via videos I can at least follow races a bit