There are a number of contributing factors:-. .. .The tail boom is relatively long and is vulnerable to a bending moment, caused either by downward decelerative forces at impact or by contact with the ground on rolling over; if the tail boom has been struck on impact then it will at least be weakened if not severed. Contributing factors to rollovers are the relatively high C of G with engines and gearbox often above the cabin, and the fact that run-ons are often not straight. Once the aircraft begins to roll the main blades will touch the ground and their sudden deceleration will cause a turning force on the airframe again swinging the tail boom around. . .. .There's an awful lot of energy looking for somewhere to go when it happens - definitely best viewed from a distance