Divorce Law - Is the Dog a Crime?

The man goes to the ex wife's house, knocked on the door, nobody answered. The man could hear the dog barking. He decided to take the dog back to his house. To his surprise, the door was unlocked. He opened the door, dog greeted him. At that moment, he felt bad for his ex wife and de to leave without taking the dog, he was charged with burglary. I guess is that just breaking and entering, or not breaking what? What's the crime here? Actually, i don't even get how this would be any kind of crime. Possibly, like a trespassing type thing. But he didn't take him, he didn't took the dog