that could have been because of shifting from equatorial to polar position tho. they wouldn't have been prepared for it. even if the place doesn't get washed away the changed temperatures will probably kill off most food sources growing and living there.
also it's telling that there is evidence, that wasn't washed away or buried deeply, of the existence of these people. they just would have been living in a lush high altitude plateau in the warm subtropical zone and then are suddenly temperate zone. everything freezes in winter and nothing was prepared for it, except a few outliers and burrowing animals and many trees would not survive that transition very well. though, ok, a lot of things would make it.
point being that there probably was no food for some years while herds built back up and people got adapted to the new weather and adjusted their construction to adapt.