This self-destructing rear derailleur typically happens under high pedal stress, sometimes you will get a warning with the derailluer brushing the spokes on the inside gear before you mash the pedals. You wouldn't neccessarily have seen any shifting issues. It is possible to set this condition up with a derailleur bent by falling over, but this normally gives shifting issues, and of course you would know if you had hit it. Sounds like LBS is ducking for cover.