a dog and living in coyote country.

1. Watching the videos gave him the idea that it's okay to express himself that way.

2. He is hearing distant coyotes that are difficult for humans to pick up. Once a pack of coyotes gets going (usually after a kill, unfortunately) it is a sound to behold.

That is the #1 reason my dog howls or barks between sunset and sunrise.

#2 is nocturnal visitors in the yard, specifically our friendly neighborhood pack of javelinas.

Maybe distant sirens? Humans usually pick them up, but it's possible they could remain on the outskirts of audibility, far enough away that they are difficult to hear. The fact that it is happening at night makes me suspect, like you, that it's animal activity.