The boat was a fin-keeled fibreglass Westerly GK29, 29' long, around 3-4 tons in weight. They've been around since the 70s.
A yacht like that should be stored ashore resting on it's keel in an adjustable steel cradle giving secure support under and around the hull to stop movement. Additional props would be needed bow and stern to keep it absolutely secure, and braced like that, it should be able to survive winds of 100 mph plus, even with the mast up, and genoa furled, as in this case.
The fact the owner was present when it moved suggests he was working on the boat, perhaps antifouling, and may have accidentally or deliberately moved one of the supports (something that is banned in most boatyards). I'm sure an investigation will show if it's a failure of the supporting cradle, or the owner's activities that caused the tragedy.
While my heart goes out to the deceased and family at this time, for club members working on boats still, please DON'T move any supports for maintenance. Leave it to the professionals, or wait until launching time for a last minute touch-up of the "patches".