There are some unverified 'theories' about this.
* Dwarfs and goblins of old used to dig deep under mountains and hills and rob or 'burglar' the noble minerals, carrying them off to their boroughs. They were at first called 'bergers', changed to burglars.
* The old chieftains used to have themselves buried with all their treasures under a mound, which word even being of obscure origin is sort of a mountain.
Those mounds were burglared all the time, cause everybody knew the story.
From: The Mountain and the Thief.