Can anyone explain, or have any thoughts, about how this could've happened?
It's a "UAC" brand compressor I got at rockauto.com - worked fine for about 10 months and the pulley still rotates smoothly, but the springs were destroyed, see pic:

 

I'm going to say either compressor seized or overloaded.

What caused the original compressor to fail?

 

Thanks Ali I noticed in the box of parts I dropped off to the mechanic who did the installation that the desiccant wasn't installed. So, most likely, there was excess moisture in the system - unfortunately my warranty expired before he installed the compressor.

 

Ah, Rockauto, purveyor of Chinesium