This is a Blue Hubbard squash. Leaves started turning yellow a couple weeks back.
The crown just came apart with very little force, and other parts of the vine that were "rooted" seem to be similiarly rotting. The closer to the original crown, the more rotted.
After checking with the folks at my organic gardening board this has been ruled damage from Squash Vine Borer. Guess I'll add that to my list of things to work on next year!