Well, when I was 16 and we first started renting DVDs, there was a friend of mine (which was 16 too) which used to do the same thing your mother did.
At the end of a DVD, he was just re-winding the chapters in reverse order because "the rental guy always told me to rewind the VHS before giving it bach, so why shouldn't I do it with the DVD?"
I wish I had my first camera with me to film this when another friend of mine ejected the DVD once finished, put it back in the player and (looking at the "DVD-rewinding guy" while trying not to laugh) just said "see? it already IS at the beginning".
I still don't know how he managed to stay calm while doing this, while the face of the DVD-rewinding friend was just priceless.