Rear Suspension problems of the 2000 BMW 3 Series

While driving on a smooth road, the rear wheels locked up - without any warning. Car came to a sudden unexpected halt - on it's own. This situation would certainly have resulted in injury/death, had it occured on a highway or other heavily traveled road. Dealer advises that the suspension ripped clear off the frame of the car. Vehicle is not driveable. Have filed a complaint with BMW, but they say it's not covered under warranty - despite this being a 'certified pre-owned BMW'. Car is just sitting at hunterdon BMW in lebanon,NJ.