The farrier told me that because her toes had been permitted to grow out too long for a while, blood vessels had extended farther than they otherwise ought to. (Sort of the same idea as the quick extending farther down the nails of dogs or cats who go a while without their nails being trimmed regularly?) Does this explanation make sense? He picked up the trimmings from both of her hind toes and compared them for me to show me that he hadn't taken any more off of the hoof that started bleeding than he had taken off the other one, which was fine.
I was watching the whole time, and my mare did not react at all when he nipped and she started bleeding -- not even a flinch. She didn't show any hesitation to put the foot back down and rest her full weight on it... and she walked completely sound immediately after. She has been completely sound all this week too. So, it didn't seem to cause her any pain... but I'm still sort of wondering how "normal" this is? I've never seen a horse start bleeding while being trimmed before?