We know he has been under heavy pressure from the Board and the VCs for some time to get his crap together. It looks to me like things finally came to a head and, considering JW's poorly managed temper, I can only imagine how this all went down.
I agree with Ira that there must be one hell of a back-story here. Also, let's not forget that Witchel is the one who has been running Prosper day-to-day since the beginning.
This development is extremely noteworthy.
ETA: We are (and perhaps rightfully so) focusing our discussion on Witchel, but I do think it's important to keep in mind that Witchel's departure appears to have coincided with the departure of Prosper's top information security and fraud prevention executive.