To quantify my response below - I am a holder of a CPC (certificate of professional competence) in road freight management.
If an item is properly secured it shouldn't move in normal circumstances in any direction.
The greatest force on said item is under heavy de-acceleration (i.e. heavy breaking or crashing) is forwards.
In this situation the item, if it breaks loose, could come through the rear screen and hit the occupants on the ack of the head.
On a lorry you should have the load against the bulk head just behind the cap, and if there is a gap you should fill the gap to stop the load moving forward. Not to do so could result in a fine, or worse, your load through the cap!!!
In a nut shell, your luggage shouldn't move if properly secured, if it isn't you don't want it in the back of your head!
Home to black Alfa Romeo 159 3.2 V6 Q4 ,green MGF VVC and red MG Maestro T16.
MG - the friendly marque.