The story of the Gyeongju artifact discovered during sewer construction

It is presumed that a major incident suddenly occurred among the monks, so they gathered all their important items in one place and buried them.

There are relics from the Silla and Goryeo Dynasties, but there are none from the Joseon Dynasty.

It can be assumed that a major incident occurred in Gyeongju during the Goryeo Dynasty and the relics were hastily buried in the ground.

And the fact that it could not be taken out again and was only discovered now is

All the monks who knew the location of the burial were massacred.

Then, who is the culprit?

Why or Mongolia?