Village attacked and destroyed while most of the men were out hunting or warring?
Or perhaps few men left in the village due to recent fighting.
Since they were doing large scale human sacrifice—chances are good they did the same thing as the Aztecs. They sacrificed people they captured in battle. That means that they would be surrounded by mortal enemies—just like the Aztecs.
It would only take a generation or two of success and comfort to make them weak and vulnerable. At that point their boundary enemies would gang up and burn down the lot of them.
Its pretty much a regular pattern around the world.
There is no evidence at all of outsiders ever taking Cahokia.
This was almost certainly a massacre perpetrated by locals. The most logical explanation being a funeral ceremony. Many, many cultures have had a history of sending the slaves of the rulers with them when they die.