Why did my dog start howling while alone?

He's lonely. Dogs howl to communicate with other dogs, like wolves or coyotes do. He's crying out for companionship. I would suggest a pet walker or a dog daycare a couple times a week, to soothe the loneliness and tire him out. Like people, we may be fine with certain things for years then suddenly no longer can handle them. We change. He's trying to talk to you! I bet if you do these things, it will taper off or stop.