Ultegra Rear Derailler Failure(long)

On a recent road ride, after approx. 48km of riding, I noticed what sounded
like my chain popping off from the front chain ring while coming to a stop.
To my horror when I stopped suddenly and looked over my bike, my rear
derailleur had actually SNAPPED in two, breaking at the lower pivot(?), just
below where it says Ultegra on the derailleur facing. The lower half of the
derailleur was wedged into my spokes with the chain still attached to the
small rings and the upper half was still attached to the derailleur hanger
which was bent inward quite badly.

This blows me away as I wasn't even under heavy pedaling load when this
failure happened, I was gearing down for the stop. The derailleur is about 4
years old, an RD-6500 9-speed. My bike is a Colnago Asso with a badly
damaged hanger and slightly bent chain stay. All I can ask is "What gives??"

Has this ever happened to anyone else in forum land?
Thanks
Joel
ps I thought this was posted yesterday, but I did not see it on its own??

I've seen this happen. I was behind him at the time. A friend had just
fallen off his bike. He got back on, his derailleur hanger had bent
inwards, he changed down, the derailleur went into the spokes and bang.

It is highly probably that the hanger was bent BEFORE it happened.
Othewise I don't think it could, as it would take a lot of force.
Had you fallen off, crashed?

The hanger must have been bent prior to this happening. But I did not fall
and I usually take care not to damage my gear during travel, etc. Whatever
the reason for the bent hanger, I have learned of one more check to make
before that next ride.

Joel
wrote in message
oups.com...
I've seen this happen. I was behind him at the time. A friend had just
fallen off his bike. He got back on, his derailleur hanger had bent
inwards, he changed down, the derailleur went into the spokes and bang.

It is highly probably that the hanger was bent BEFORE it happened.
Othewise I don't think it could, as it would take a lot of force.
Had you fallen off, crashed?