Well I agree and disagree with some of this bill. I've owned a few pure whippets, and a few dogs with a fair amount of whippet in, and NONE of them yapped when in pursuit, ever.
Personally, I believe it's not any particular type or cross or whatever, but purely frustration more than anything else. Age of starting the youngster, proximity to cover when slipped, and more than anything else, in my opinion, starting them alongside another dog. You see it even when just walking two youngsters together, one will always try to be in front, competition sort if thing