Overland Park, we have a problem...

I don't know what is wrong with my Cody Bear. He has never done this before. I am shocked, I am freaked out and I am worried. What is going on, you ask? He has started to hump his brother Cooper. Now I know that this is a normal thing for dogs. BUT it usually is something that starts early on. Cody is 3 almost 3 1/2. He has always humped blankets which he balls up and puts underneath him. He loves one blanket in particular, a KU blanket, we call it his girlfriend. Well, he has never humped anything else in his life. Nothing, nada, no one. So imagine my surprise when I see him mounting Cooper from behind yesterday as we watched a movie. At first I thought I was just imagining it and they were just playing around like they do. They jump all over each other all day long. But then I saw it...the back and forth action, the front paws clutched beneath Cooper's stomach. Did I yell? Did I say "no!"? Nope...I calmly said "look what Cody is doing to Cooper." to my husband. He freaked out and thought I was weird for not stopping it. I just wanted him to see it so he knew I wasn't making it up. After he saw, I told Cody to stop. He did. Then he started again. Like he had found a new toy that he couldn't stop playing with. He mounted his brother again. Cooper had this look on his face like "whatcha doin Cody?" Not too bothered and not too worried about what was going on. Just happy that they were playing. This time I clapped and told him more sternly "NO!" He stopped. He heavily panted for a few minutes while I got back into the movie and then he did it again. Just walked on over to Cooper, put his paws on his back, and started humping. "NO CODY NO!" I yelled. That time it worked. He hasn't tried it since. But my question is, why the sudden urge? He is neutered, they both are. Is this what they do to pass the time when we're at work? Cooper sure didn't seem to mind. Oh boy. I do not like this. They say it's a dominance thing but Cody is clearly alpha, always has been and Cooper submits to him all day long so why would he suddenly feel the urge to show his dominance in this way? Any ideas?