I thought it had to do with the Dragon Priests. The Druagr were dragon worshiping Nords, no? The return of the Dragons brough the return of their Dragon Priests and Druagr, IMO.
It's this sort of thing I was considering. The ancient nords worshipped dragons, and after their return arise to answer their call and serve them again. Doesn't seem too unlikely.
An alternate theory: Ancient Mages protected the tombs with curses and spells so that if a grave robber should attempt to enter the tomb and steal it's treasures, the dead would awaken and defend it.