I was hoping to avoid getting down to specific cases and was trying to keep things tried to keep things on a general level but since you ask.
It's difficult to see from the photo but the rock looks like limestone, the patterns look like the patterns you typically get on exposed limestone outcrops. So I would guess that the bullaun may possibly have started life as a solution/erosion feature that was subsequently enhanced into its current lovely bullaunic (my invention) form.
I may be totally wrong but that's my best guess from the photo.
Good luck with the new moon and your beginnings
fitz