How did I fry this drive?

on a wider note, on the law of averages, it's just a matter of time before some form of hardware will end it's life whilst being examined in the lab. No matter how careful you are with your own procedures, hardware does not last forever.
I had a PC last month where I powered up to check the BIOS settings and "pop" went the power supply with a nasty burnt smell. I went through my procedures and kit and came to the conclusion it was just "one of those things". I swapped it out for a replacement (had to really to get the BIOS details) and left it in the new one as a good will gesture (soft in hindsight).