The Yetis are losing their homelands as humans encroach on their Himalayan environs. They have decided to take over the Santa Lands as their territories.
This shouldn't be too much of a stretch for anybody. High fantasy around the myth of Santa Claus -- SPOILER! -- is nothing new. Since his remaking from Saint Nicholas into a jolly old wizard of gift giving for Yule Time cheer that's been going on for centuries, folks have been making Gary Gygax look staid as a fantasist. Reindeer landing on roofs and a velvet clad man sliding down a chimney in the middle of winter is just the beginning. The North Pole wasn't just a place for explorer sailors to die, it became Santa's home and where his workshop was. Elves and a magical missus would pop up to explain the impossible productivity going on there. The age of TV would take things into fantastic realms of science fiction and the Wild West. Santa would take on the Martians of Mars and help out the fictitious version of Billy the Kid. As far as I know yetis are not yet that much of a staple for the Christmas Holiday, but ever since I watched the Rudolph the Red-Nosed Reindeer special with the Abominable Snow Man, I can't live without them.