These aren't oribatids, but Astigmata in the family Acaridae, probably the genus Sancassania. They commonly feed on dead arthropod carcasses where large populations build up and consume the soft tissue. See big fat females and lots of small juveniles in one photo. Next generation deutonymphs then disperse back into the soil and seek another habitat patch.

Thanks for providing the species name; my students have some research to do. We have observed three instances of an isopod eating a mite and once an isopod laying on its back, rolling a dead young isopod around to get to mites. The students isolated this isopod and a few days later, observed it was dead and being devoured by dozens of mites!