I've seen this happen before. If you check the scene from overhead, there is probably some overlap of the top parts of the tree. When you move them further apart, it will stop. At least it does for me. For some reason, from the instances I've seen, it usually does this on duplicated or replicated trees of about the same height. You can shrink a tree to a height lower than the branches of the tree and stick it under the branches of the larger tree without problem. It just seems to happen when any part of two trees overlap anywhere.

This is not my "second childhood". I'm not finished with the first one yet.

Time flies like an arrow; fruit flies like a banana.

"I'd like to die peacefully in my sleep like my grandfather....not screaming in terror like the passengers on his bus." - Jack Handy