The shaman’s union pension fund bankrupted them. The tribe moved to Florida.
“shaman’s union”
That’s a good tongue in cheek way to express a strong possibility. As with the Maya, so (perhaps) with the Cahokies: closely-held esoteric knowledge by ruling priests becomes vulnerable when they are rounded up and murdered (a la the prophets of Baal when Elijah showed them who’s boss).
Into the grave (or pyre) with them goes all that esoteric knowledge on which the whole operation of society was based; then people scatter, scramble and hustle for themselves. Being mostly slaves, they knew how to do stuff, but would’ve had a disdain for the superstructures of former oppression.
A second tongue in cheek theory might be that they tried an open borders policy and were invaded by a massive influx of peons from Mexico who gladly took what wasn’t theirs until, voila!, nobody had anything (and, no, they didn’t enjoy it).