I (as well of the rest of the PC-12 pilot community) have followed this crash closely since it happened. The KFC-325 autopilot on this PC-12, as well as the rest of the "Legacy" PC-12's (c/n 101-886) tends to disconnect in anything worse than light/moderate(ish) turbulence most of the time. It will hold some times in Moderate turbulence, but any good jolt usually disconnects it (this can vary from airplane to airplane). He was in IMC, near some decent weather, so the chances he was getting some good turbulence/chop are pretty good.
The other plausible explanation for the disconnect was, based off of his groundspeed, he may have been climbing in "VS-Vertical speed" at a set rate per minute. With this mode engaged, you can climb all day long in a set feet-per-minute climb....until the airspeed drops too low and you activate the shaker. Once that happens, the autopilot disconnects. Keep it there, you get a second shake, increase the AOA and you get a push.
Either way, this guy (who, by all accounts, was extremely inexperienced in both the conditions as well as the plane) allowed his airplane to roll over to the right, enter a dive, and get the airplane speed up to 100 KIAS OVER the published VNE!
Interesting side note; SimCom in Phoenix, AZ where he trained, stated that his flying was ok and they signed him off. His sim partner however, stated that he was way behind the curve and should have never been signed off. It will be interesting to see what (if any) lawsuits come from this.