I like the mystery. It gets the wheels turning.
Perhaps the corpse went the way of Yoda or "Ascended" and is now trying to look out for its descendants. Being non-corporeal, it is having a devil of a time. What luck for it that the adventurers caught its attention. Now it will follow them around, trying to get them to listen (unintentionally causing havoc in the process) until they agree to help sort out some injustice for its distant progeny.
I am also quite partial to the mystery of this submission. My addition is that when I think of a crypt it is usuall one for an entire family. Imagine a crypt intended for several generations completely devoid of bodies. Perhaps the family had some sort of special link to the underworld that made them particularly desierable as undead. Perhaps the crypt is so ancient absolutely everything has been eaten away by time. New Orleans buries their dead above ground to prevent the corpses from floating about during floods, perhaps something similar (or the reverse) happened.