In the 14th century western Europe was a meteorological mess. France was having way too much rain and it was cold. Crops failed again and again. The 1400s reversed all that and things got most excellent again.
But Cahokia apparently didn’t have bad weather or crop failures. I always assumed that they were having a bad time too, but no.
Maybe the population was getting too big for the available hoof protein, and they left for greener pastures.
Their dispersal is apparently what gave us all the different tribes of the US plains and Midwest.................