Any arrangement that does not have a solid bracket at the rear to prevent items slipping over the back is a recipe for potential disaster.
The cause is obvious.
Not so obvious to me.
Are you saying that something still tied to the bike fell over the back, and since it was still attached didn't tumble down the road but rather wedged itself up into between the rear tire and the sub frame effectively locking up the rear wheel and causing a skid?
Did you not use the saddlebags so you could tie things up to their mounts instead, or did they just fly off at impact and out of the photo(s) frame?