They have a cultural taboo against interacting with Coyote.

This taboo is enforced with legends/examples of what happened to people who broke the taboo. The point of the stories is that, if you do this, bad things happen to you and to the people around you and it will be your fault.

Annie's actions as medium were part of her job. Technically, that means she didn't break the taboo. But, she was having tons more to do with Coyote than any of these elves would have. That made her suspect before things went wrong.

When things went wrong, Annie fit the criteria of "person to blame." That makes her the scapegoat.

The thing about scapegoats is that they don't have to be at fault they just have to be a person who can be blamed. These people have lost their homes and everything familiar to them. They're angry and upset and need someone to blame.

It's a more human reaction than we've seen from the people in the forest up till now. It fits how the story has been developing. Previously, the forest dwellers were, to some degree, a bit idealized. Despite Coyote being creepy and sometimes terrifying, the forest people overall seemed better and nicer than the court (which can also be creepy and terrifying). They seemed to accept Annie as one of their own. Now, we're seeing an understandable but less pleasant side of them.