Even properly trained pit bulls can be dangerous. See my post above.
Their unpredictable behavior is natural to them and developed over centuries. Training certainly helps, but it only takes one incident for trouble to pop up. Proper training only gets them so far. At some point, their natural instincts can overwhelm any training, no matter how good.
Any properly trained dogs can be dangerous, not just pit bulls.
The reason there are a lot of pit attacks is because they are the most abused dogs in the world, and insanely overpopulated. I would not be surprised if Prescott's dog is a shelter dog that was abused like many other pit bulls.
Statistically, Chihuahuas, Dachshunds, Chow Chows, Doberman Pinschers, Dalmatians, Rottweilers, Jack Russell Terriers, and German Shepherds are more likely to carry "aggressive" traits than pit bulls, on an individual basis. But when a Chihuahua bites someone, it obviously isn't going to do the damage of a giant, strong-jawed pitbull.