When the transport securing was released, the wall fell out of the flatbed. During shunting, the wall had slid backward and, as a result, was not individually secured by the buckets in the gate.
Backward slippage is prevented by securing the load with chains, posts, and/or blocks. Given the changing weather conditions, etc., the friction also varies. There is always a risk that a wall could slip out of the buckets when handling flatbeds, whether they are swing-out or tilt-out types. It should therefore always be standard procedure to ensure that the walls are securely fastened to the buckets during loading and unloading.
