Something other than genetics is going on with bullbreeds and it can only be an environmental factor.
Many of the bull breeds (including Staffys and Pit Bulls) have long and complex breeding intentions in the lineage. They were, in many cases, bred to be a working class man's fighting dog that had to live within the family home day-to-day as well as engage in 'sporting' fights (the working class not being able to afford to maintain kennels). To this end they were selected to be companionable towards humans, and beyond - they typically crave human contact and suffer very badly psychologically when emotionally neglected.
The environmental trigger you're looking for is a change in societal family structure. Bull breeds should only be kept by owners that can be with them all or most of the day on all or most days of the week. If they're left on their own regularly for long periods, or pushed out into the garden/backyard to look after themselves, they will become frustrated and will start to exhibit widely varying behaviours in order to get back the emotional attention they need.
When you look at the media reports of bull breed attacks and the wider circumstances you can see broad patterns emerging:
- Status dogs trained to be or act aggressively. Surprisingly rare, I suspect most people buying status dogs aren't that well clued up on dog training. And typical security dog training is all about the appearance of aggression to the observer and making this a game for the dog.
- Emotionally neglected dogs that have been kept on their own outdoors in a backyard, or left alone in the home repeatedly whilst the owners are out, or emotionally displaced by changes in the family structure/routine. These are the most common and most tragic cases.
It's not just bull breeds that suffer from this type of neglect, but in general they are less emotionally independent than many other breeds. And when it comes to emotional displacement, e.g. the arrival of a new baby, then no dog of any breed should be entirely trusted.