Strange puppy behavior

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I've bragged about my dog in several other discussions, so I felt like starting one about one of the things he does that I find incredibly strange but funny. Now, whenever I'm sitting in front of my computer (which is most of it actually) my dog would be either sleeping in my bed, which he's allowed to unless I'm on it, or by my feet. The room leads to a long hallway/patio, on one end of the hallway is the front door, the other end leads to a common room with doors to the kitchen, bathroom and a back yard/garden. During the day, whenever I leave my chair, the dog would wag his tail, look at me, then run towards the front door, expecting to take a walk. This is very normal, he likes to go outside and he figures if I'm leaving my chair it might mean I'll take him out. Plus, I take him out several times a day, so like I said, it's very normal and obvious. However, as soon night falls, this behavior changes entirely. Instead of running to the front door waging his tail, he will run to the backyard, barking like crazy. If I went to the bathroom for instance, he would remain at the backyard barking until he'd hear the bathroom door closing. In which case he would come back looking at me, then turn around, bark a bit more and then he would go back to sleep on my bed, waging his tail and for some reason, I'm sure feeling all proud about himself. He really does look that way, chest puffed out,walking tall. Kinda like when we play fetch and I praise him for grabbing whatever I throw and he droping it at my feet. What do you think? Is it because I praised him for making cats run away from the yard in the past? If so, why does he do it ONLY during night time?. Is it because during the night the only places I go to are the Bathroom/kitchen/backyard and he feels the need to go and say "Go away, my master is passing by!". Is he just nuts? Oh! And please, share your stories about whatever nutty things your dogs do. I'd love to hear them. Rich