There is a large range in what different people call a medium rat, so it is possible that the rat you gave the snake was quite a bit bigger than what the previous owner had been feeding it. Even so, that should not have caused the leg to get stuck like what you are describing.

I have never purchased frozen feeders from Petco or Petsmart, but I have heard that sometimes the quality is really bad. Perhaps there was something like freezer burn that caused the leg to be stiff and unable to just fold back like they usually do.

Glad everything turned out ok! In the future, I'd feed this guy smaller meals. Not so much because I think a repeat of this problem is likely, but as others have mentioned, it sounds like more food than an adult male needs.