Even if they are hominid fossils. 3-5 million year old hominid fossils??
Seems like a very long time of undisturbed preservation.
Probably volcanic deposition which can cover strata forever unless erosion or continental movement shifts the topography. The fossils at Olduvai Gorge were preserved for millions of years until the changes in surface conditions exposed them. Even found fossilized footprints at Laetoli made by Australopithecenes.