It is not horse meat as such, but horse meat that wasn't meant for consumption. If you are keeping animals that are going to be sold to food market, you have to comply with some regulations about how you keep the animals, what medications they can receive etc. Some horses are kept for other reasons than food market, so their owners would not necessarily worry about the medications the horse receives and if it then get into the food market, it is not good for people. At least this is what was said when there was the wrong meat labels scandal in the UK a few year ago.
https://en.m.wikipedia.org/wiki/2013_horse_meat_scandal