“intentionally burning thousands of their houses.”
Perhaps this was a plan to exterminate vermin like rats, mice, fleas and lice. Supplemented with a good bath and plenty of nit picking, it could have been quite effective. If there was a disease epidemic they might have done this to appease the gods and discovered that it also eliminated disease for a time.
The circle design reminds me of similar marks carved into early celtic/british stone dwellings. Also the circle sites remind me a little of stonehenge.
Once in fifty or seventy years the settlements were abandoned and the people moved elsewhere, burning down the abandoned settlement...
that's a long time between pest-control treatments!