20 wolves are a very large pack. They probably started to attack him because of their kill on the ice, they saw him as a threat and were protecting their feast. I’m not sure why he started shooting at them though, he made it to the cabin’s safety, and they would most likely leave after they were full. 20 wolves can make a young moose disappear pretty quickly. But I’m sure he made good use of the fur, so it wasn’t all a waste of ammo.
“I’m not sure why he started shooting at them though”
It clearly states why in the article.