Animals with antlers grow new antlers each year.  While antlers are growing, they are covered with a vascular membrane called "velvet", which provides nourishment to the growing antler.  Once the antler has grown and hardened, the velvet is shed in late summer, which is what is happening in this photo.  The animal will then shed the entire antler in early winter (for male caribou) or early spring (for female caribou).  A new antler will then start growing in the spring.  All antlered animals including deer, moose, elk and caribou have this (although caribou/reindeer is the only species where females have antlers - the rest only males have them).  Antler growth and shedding is regulated by the natural cycling of sex hormones, so it isn't really anything more disturbing than a menstrual cycle.