Cheap junk tool made by the lowest bidder with poor quality control.
Most sockets are cold worked from blanks and pressed into shape, then as Tomcat says they then go through a heat-treatment process which makes them brittle, but also very hard (more brittle=more hard).
I don't think they anneal them to soften them again as the hardness of the socket is desired, as you want your tool to be harder than the bolt in most cases. (Though if it was an impact socket these are softer so when you use an impact driver they wont shatter and send metal splinters into your eyes.)
So my prognosis is a combination of:
- Low quality metal
- Manufacturing defect (Struck off center, giving it too thin of a sidewall on one side or something similar.)
- Poor design
Probably 95% of the time these sockets work fine for the light duty they are subjected to, rest of the time they break like yours did.
Just go buy a tire-iron from NAPA or CarQuest or what-have-you.