My thoughts are that the tombs date back to very ancient days, and that successive cultures added more to the structure, with long periods intervening between levels. Then, some decades back, a powerful mage found references to the place buried in some disused section of a great library. When he made the fateful decision to take the step to become a lich, he naturally thought of this place and sought it out. After the went through the terrible process, be began using the raw material available to him to create various forms of undead, going so far as importing a few for his own amusement, then finally arranging for the entrance to become sufficiently uncovered that his minions could throw open the doors and begin terrorizing the surrounding countryside. He envisions a night where the world belongs to him; where all living beings are but cattle to feed his soldiers.
Thanks for the correction!