The yetis have become climate change refugees, forced to leave their traditional ranges, and this has brought them into conflict with bears.
Some (obviously unscientific) people confuse the yetis for wood goblins:
“Folk beliefs say that the wood goblin is the master of the woods. All animals, even bears, submit to him. The wood goblin has a strong hypnotic power, thus he is not afraid of any animal.”
The yetis are normally a gentle species, building pyramids (presumably in memory of their time as the Atlantean slave class that built the Egyptian pyramids) and chatting telepathically with all and sundry (but not, obviously, bears):
“They make strange pyramidal constructions of trunks and branches in the wood – sometimes 3 or 4 meters, sometimes only 30 cm high."
[I]t seems that local residents have already found a common language with the yetis – they leave candies for them and communicate with them mentally – yetis are believed to be telepathic. Igor Burtsev even claims that to a certain extent, yetis can imitate the human language. “I would, without doubt, call the yeti another species of man,” he says.
EoR does, however, wonder why no one has investigated the immediately obvious explanation: that these are the descendants of escaped or abandoned humanzees from Ilya Ivanovich Ivanov's experiments in the early twentieth century.
(The Great Intelligence)