How did this happen?
 

just want to ask you guys how can i prevent this from happening again?! I have no idea how did this happened? I climbed steeper hills than this one...

I was going uphill when this happened... RD was detached from the RD hanger, RD Hanger was bent, the chain got stucked on the RD. RD was stuck on one of the spoke Now i got to jog for exercise for a while... until i can find 10 speed RD here... (it seems that local shops here now only sells 11speed RD)

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It happened because ...

Derailleur bolt was loose - prevent it by checking derailleur tightness during routine mainenance
Down shifted under load - prevent it by spinning lightly and relaxing pressure during the moment of shift
Derailleur hanger was previously bent, or loose, contributing to inward movement of RD - prevent it by inspection and repair as part of routine maintenance.

You'll need a new derailleur hanger, a new RD, and a new chain. Have the dropout/hanger alignment checked by a shop with the correct tool. Check your spokes for damage or uneven tension where the cage struck the spokes - you might want to true up and have the wheel retensioned.

Good luck.

i see. might probably be downshift under load thing... thank you for the analysis. now i have to rest my leg for awhile and search for a 10speed rd in our LBS.

Looks like a chain link pin popped out, then the chain jammed in the derailer.

Went through this exercise recently when a rear blinkie fell off and got caught in the chain just as I started out from a dead stop. Not only did it bend the RD but bent the removable derailleur hangar. I knew the LBSs in my area don't carry the specific Shimano RD I needed (10 speed extra wide range) but it only took about 5 minutes to find a new one on ebay. Had the hangar in 3 days and RD in 5, both at prices similar to what it would have cost at the LBS if they had one in stock. All you need to know is the make and model to do a search online. Much less effort than checking with each LBS in my area and hoping for the best.

If your derailleur hanger is aluminum and it got bent then you should replace it. Don't try to bend aluminum back into shape, because bending it will weaken it and it will break. Steel can be bent back into shape, aluminium should be replaced.

There's this new-fangled thing called the inty-net. I hear you can order just about anying....even ancient obsolete derailers from 2013! Usually cheaper than what you'd pay for 'em at the local bike monger's, too! :)

Please make sure your limit screws are set properly so this doesn't happen again.