Pure speculation! However, this was a wide-spread occurrence not just one or two cows hinting at some systematic setup. I also don't remember the details as it was many years ago and I was quite young - it's possible that there was a pair of legs.
Forgive me, for my biology is a bit rusty.
A gene can become more common in a population without being selected for. However, invoking random genetic drift as an explanation is generally dirty pool, epistemically speaking. We should expect a gene that creates extra useless legs to be selected against. (Nutrients and energy spent maintaining the leg could be better used, the leg becomes more space for parasite invasion, etc.) Assuming that you were dealing with such cattle, you should assume that some humans were selecting for them. (No reason necessary. Hum...
If it's worth saying, but not worth its own post, even in Discussion, it goes here.