Sorry for your bad breaks (pun not intended).
I don't know why a chainstay should break, but this 2nd break shouldn't have happened. The breakaway hanger should have prevented it, and sadly let you down.
I'll spare everybody the soapbox, and just wish you better luck the next time around.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.