My pet theory is that humans captured wolf pups, possibly by dealing with parents first, and kept them around as pets. People love playing with tiger, bear, and wolf pups and keeping them as pets today.
It would take generations of breeding the tamest ones, with the behaviors you wanted, to get something like the beginnings of domesticated dogs.
It also makes me wonder about the longlasting question of speciation. If it happens suddenly, shouldn't that indicate a singular (or near-singular) instance of mutation?
". Within just 15 generations of selective breeding, the experiment had yielded foxes that could live with people."