Musk Oxen

During the fall rutting season watching a Musk Ox could be delightful. During this period of time, male musk oxen fight for the control of harems. The fight is so powerful that when they bash their heads with each other the reaction force can shake the ground which could be felt from a long distance. They are really very old species who have been living on this plane for over 187,000 years. In the 1800s, this species was extirpated from Alaska, but again in 1930s, they were reintroduced to the world as their population saw a spike. At present, there are over thousands of musk oxen roaming in Alaska. But due to external interference, their population have been declining. What is the reason behind that?