Had a wtf? this morning - Ping AMuzi

Went out for my morning ride. Just before a rather steep hill changed
down and I heard a clump and the chain jammed.

I looked back and half of my rear derailleur was sitting on the
asphalt.

OMG, I thought to myself, this is almost as embarrassing as a crank
arm falling off. Then faced the "walk of shame" home.

Note, I scrub chain/jockey wheels/sprockets with water and detergent
every 200 Km. Was almost due for a new clean. Gear change was
absolutely smooth until the incident. HI and LOW screws perfect, no
way the chain came off and "fell into the spokes".

Note I managed to rip a spoke clear out of the wheel with the
breakage. What fell into the spokes was half the derailleur. I suppose
that'll mean a new wheel too

:(

<https://www.sendspace.com/filegroup/v/qdHDWfYEJvo99L3/MhFp6hhZL5Z8ugxHfSuN/uTo4>

The "cleaned" in the filename is because I cleaned the EXIF metadata,
not the parts after the incident....

So "wtf" happened, what did I do wrong and how can I prevent it from
happening again?

TIA
--
Don't be evil - Google 2004
We have a new policy  - Google 2012
Google Fuchsia - 2021