I'll be curious if others have any suggestions, but that is pretty darn close to my set up (I think I am -1 and +2).

Looking at the images, it looks like it grabbed onto the snow directly behind the initial subject. Not sure why. The -2 on sensitivity should be making it stay on subject, and it could have grabbed the tree. But it does seem like it is the snow behind the tree that it grabbed onto.

So, I would chalk it up to a very difficult subject. The falling snow against the background is a bit too much like the owl, IMO.