go between the bark and the tree

To be overly involved in someone's personal matters—often those of a married person or couple. You shouldn't spend so much time alone with Elaine—she is a married woman, and you're going between the bark and the tree! You know far too much about their relationship and have definitely gone between the bark and the tree now. I can't believe those two are getting divorced. I really hope that someone didn't go between the bark and the tree.