Two things come to mind,
1. I know the whole back seat thing has been said but this can cause the foot to shift forward in the boot. This leads to the second issue.
2. boots are too big. You foot is shifting forward in the boot thus too much room. If you have had the boots fitted it might be time for a re-look. The liners could have packed out and are not holding in the boot correctly. Check out a new liner or some foot beds to take up some volume and keep that foot in place.