Icing is being mentioned as the culprit for this terrible crash. Matter of fact, it looks like tailplane icing, at least it fits the description (listen to the first minute of the video). God be with the victims' families in their time of grief.
Based on information coming from the NTSB several things appear to be established, to wit, the plane was on autopilot during icing conditions which the crew had mentioned on the CVR. At some point the AP automatically disengaged into a stick shaker event, meaning an impending stall. There was a severe nose/down pitch event, but it's not clear which came first or whether either were in response to pilot reaction to the stick shaker or the pusher. Pilots of this type of aircraft would have a much better idea what might have occurred and therefore we'll leave further speculation to them.
A couple of final words. It's premature to blame the pilots. For all anyone knows there could have been something wrong with the boot system or something else unrelated. As to the conspiracies being tossed out, good Lord.
Regardless of causation this was a particularly grizzly crash and my heart goes out to all those grieving right now.