I honestly don't know enough about the head roll. It sounds neurological to me, but that's little more than an educated guess.
As for the difference, reputable breeders breed from select stock to breed better animals genetically. They also provide better care for the animals prior to selling them.
The animals from the big box stores are basically bred in "puppy Mills" with little attention paid to genetics, health or care before selling the animals.
The stores themselves typically provide horrible care for the animals as well. Their husbandry is usually way off, they underfeed, etc...
Don't get me wrong, some animals from the big box stores live long, healthy lives. But more often than not when we hear about an animal suddenly dying young like that, it came from a store and not a breeder.
Considering these are living creatures who will live in our care for 20+ years, it's worth doing some research and buying from a reputable breeder.