Why do my pigs bite off each others tails?

I always look forward to interactions with one of my long time clients — Mr Makumi, a pig farmer in Nyambare, Limuru near Uplands Beacon Factory. My first encounter with Makumi was during our fifth year practical ambulatory classes; when we visited his farm. Our friendship was however cemented latter in life when I brought another friend who was preparing to get into pig farming to learn a few tips; Mr Makumi has ever since consulted me on numerous occasions on issues relating to pig health.