Well, you know, I figured that you were smart enough to access a message board and therefore could google something like the poster below you.
The Vmc roll happened at the end. Hopefully it was an auto feather issue and not something stupid like overloading the plane or loading it out of its envelope. Also, the gear come up in an engine failure scenario.
The simplest explanation is that they no longer had enough rudder to counteract the asymmetric thrust and torque of the operative engine. Whether that was due to the plane being too slow, overweight, the CG being too far aft, an auto feather malfunction or a combo thereof is to be determined.
A friend and former 350 pilot ran the numbers for me. They were within a few hundred pounds of max weight and likely loaded near aft CG given 10 persons aboard, 30 lbs of baggage each, 50lbs of baggage in the wing lockers, and enough fuel to complete the trip with no alternates and IFR reserves.