What human breeders manipulated was neotony. That is, they extended picked wolves who had an extended puppyhood. Generations of that resulted in most dogs acting like wolf puppies (not particularly agressive). Nhey never turn into adult wolves behaviorally. Then there was Cujo . . .
Our border collie mix does not really behave like a puppy. What she does is undertake, with considerable bravado, the early part of the hunt, in which the prey, whether toddlers or gerbils, is rounded up. However, once the roundup is complete, she is blessedly innocent as to what is supposed to come next.
As to what human breeders did, is such existed there is absolutely no way to know for sure, although I have given my surmise.