It could have started with a prolonged drought followed by flooding. Once it started, it just kept piling up.
This makes sense, and probably has a lot to do with it.
I wonder if reoccurring events like the earthquake that happened in the 1800s that turned the soil into liquid by liquefaction , and concurrent and ongoing events like the banks changing direction or changing course on the Mississippi? If I remember correctly, when the earthquake happened, from what little is documented of it at least, land moved like it was ocean waves, trees tossed, and were swallowed completely, And tons of destruction was done. Just little human habitation was around, so not much happened to establish communities.