My first hypothesis would be canine fovea shape; it is about vision, but it’s only indirectly about noses.
The fovea is the high-resolution bit in the centre of the human retina. While humans have a circular, central fovea, wolves have an elongated fovea-ish area that spreads laterally across the eye. Their central vision isn’t as good as a human’s, but their peripheral vision is much better; good for chasing prey. Tilting their head gets more of their fovea on target, assuming that their target is a tall skinny thing like a human.
As you move across the doggy spectrum from wolves to shitzus, their fovea gradually gets less wolf like and more human like. We’ve hyper-selected the lapdogs so much that we’re halfway to converting them into humans. That possibly explains the reduced occurrence of head-tilting in short-snouted breeds; the wolfier dogs tend to be long-nosed.
