Spiders?

Or more likely could it be when the occupants die, instead of someone new moving in they torch the place and start over?

Edit: nevermind, they suggested that further down in the article.

Huh. I was expecting they did it to destroy contagion, but that’s not in the article

Interestingly, Japan does this now. They tear down and rebuild wooden houses about every 20 years and concrete every 30. But the cause for Japan is earthquakes which I don't think would apply much here.

Clearly it's insurance fraud

Lice, mites, fleas? we were gross back then.

[…T]hey have discovered evidence of numerous ancient civilizations on the planet, all destroyed by fire, with the collapses occurring about 2,000 years apart. […E]arlier civilizations on Lagash were destroyed by people who went insane during previous [solar] eclipses and, desperate for any light source, started large fires that destroyed cities.

– Extract from the plot summary of “Nightfall” by Isaac Asimov, https://en.wikipedia.org/wiki/Nightfall_(Asimov_novelette_and_novel)