It is impossible to determine probable cause until all available information is collected.
In the meantime pilots who work in the north in the wet season should be very wary of entering cloud when imbedded thunder storms are forecast unless you have lots of good gear that can see how active the cloud ahead is. You should NEVER go near, or worse under the anvil of a developed thunder head. You need to be several miles clear. If an ATC direction is heading you into danger TELL them. They know it's not always clear skies and I've always found them very good at finding you a safer route.
If you're in a C210 and suddenly find yourself in virtually uncontrollable turbulence immediately put the gear down. Don't worry about the gear extension speed. It's only there to protect the gear doors. Better to lose the doors than the whole plane. DON'T use flap. The wing is stronger with them up.
The drag of the gear makes it harder for the aircraft to do sudden excursions beyond VNE. C210s have a lot of washout in their wings which gives them benign stalling characteristics but at high speed the wing tips have a negative angle of attack. As the aircraft goes beyond VNE more of the outer end of the wing has a negative angle of attack thus putting a large downwards force on on that outer portion. Too much speed or a combination of high speed and a sudden up aileron control input can take the download beyond the wings design envelope and it snaps off downwards (not up as is often thought) at the point where the flap and aileron meet. This is why it is so important to keep the speed under control as much as is possible in the virtually,if not completely uncontrollable conditions storms can dish up.