Apply the usual skepticism about the claims, but there's something plausible in the following. As I understand it, the assertion is that Iran basically used radio jamming techniques to force the RQ-170 into RTL mode, then overrode the GPS signal with a fake one that made it think that "home" was an Iranian field.
An excerpt from the Christian Science Monitor, a good article that discusses what may have caused the capture:
Yes ! They use the old way and it works! They used radio jamming and overrode the GPS signal.