Sounds like something that could self-perpetuate, and probably not worthwhile fighting against? Like blue is for boys (although used to be for girls: 1918: βthe generally accepted rule is pink for the boys, and blue for the girls.β).
While the photo might not be a good example, in the animal kingdom exposing your neck, like exposing your belly, in an overall non-assertive way would indicate submissiveness. It's in your interest to be submissive if you doubt you will win in combat. I'll leave the thought experiment to the reader as to whether females or males do better in combat and why this head tilt might be predominantly nature not nurture.
