Their brains roll out of the socket, and they have to tip it back in.
Every dog I’ve owned tilted his or her head. Poodles, Pugs, Bulldogs, Boston Terriers, and Sheperd/Collie Mix.
I always thought it was sort of rudimentary sonar. The dog is trying to get a fix on the location of the sound.
I’ve always called that head-tilt the “Rorp?!”
I think I got it from Scooby Doo. But it’s perfect. I even say it in real life, when I’m confused or someone just said something incomprehensible. It’s fun to use in chat, too.
ETA: Or maybe the Jetsons? Rorp?!