Could be they just moved the bones for different reasons.? I’ve also seen plots where graves were destroyed for redevelopment.
Graverobbing isn’t unknown either though.
Most likely either being moved due to redevelopment plan for the land or their lease has run out, or less likely the family is moving and bringing them with them… Most of these locations require a lease and “rent” to be paid on a periodic basis, if not paid you get evicted!
Traditionally you’d have to take out the bones and fix them up a bit every now and then. Cant remember the details.In this case they have been removed.
Recently a lot of Taiwanese moved their ancestors remains to family temples , so they don’t go to the individual grave anymore for tomb sweeping festival. It’s all kind of convenient but missing a little the excitement of the old times visiting and cleaning the grave and setting off fireworks.
I wonder if skulls are used just as a skull or religious purpose or, purely guessing, some other uses like grinding up for Chinese medicine. I know that sounds crazy but there are stranger things.