Well yes it starts with breeders and lay people that don't have the foggiest clue about genetics.
If I wanted a dog with a round head and I had 2 dogs with slightly roundish heads to breed, they'd have a litter of puppies. Of that litter there might be 2 more puppies there with rounder heads, but one of them has anger issues. Still, I want a breed with a round head so these are my best choice, so I breed them, lo and behold the next litter has 3 with rounder heads, and 2 of them have a genetic behavioural condition. My friend who is hypothetically collaborating with me on this has also done the same thing except now he has 2 males and I have 2 females, I don't know anything about his dogs temperaments, but we don't care about that. We just want the round heads.
We keep going, all this time not having anything past GCSE level biology understanding, and all the while each generation of these dogs is in more pain because of the repositioning of the organs within their face, as well as some other undesirable traits like bowed legs etc, but we don't care about that, we want the round head.
We're 8 generations in now. The gene pool is very condensed, there's been a lot of inbreeding, geriatric breeding and so on but we now have a series of dogs that all have definable characteristics within a margin, they're all 8 foot tall, red, and have got round heads and snaggle teeth. People want them because they're new and novel. They can clearly see that they can't breathe right and their legs bow outwards but they don't care. They buy them.
They paid a lot of money for my new dog breed and they want a piece of that pie. So they get their mate together and they breed their puppies who were actually from consecutive litters to the same parents. The same 2 parents with an attitude problem.
They have a litter of puppies they sell their puppies and then they are mauled to death months later.
Their puppies have matured and gone on to have other puppies. The cycle continues.
Before you know it lots of my 8 foot tall red dogs have killed thousands of people over the years but they are dogs, and nobody ever wants to imagine that a dog could be capable of ruthless and unprovoked attack let alone murder. I mean I didn't breed them to be murderers, I bred them to have round heads, a unique colour and be very tall, they just happen to be murderers so therefore they must just be misunderstood and it must be all the breeders faults.
The truth is sometimes when we play God, we make mistakes because too many people have access to things they can do without the knowledge of what they should do.
There is a lot of eugenics in breeding. It's ethically corrupt, often rushed and there isn't a breed around that doesn't have some sort of health disadvantage due to the way we have changed them. This is usually brachecephalics and breathing or mid to large dogs with hip dysplasia to name a couple.
It is now illegal to breed XL bullies. But that doesn't stop the fact the ones we have now are instinctively dangerous.