Victim stories repeatedly show that pit bulls will attack without any kind of provocation that would be obvious to people. In almost all cases, pit bull owners claim the pit bull(s) had never shown aggression before.
Pit bulls have attacked people and in some cases, killed people, for the following reasons.
View details of these attacks as well as many other triggers on Dogsbite.org.
being pregnant
clapping
dropping a glass
falling down
getting off a bus
getting the mail
handing someone a phone
hanging decorations
having a seizure
having an argument
holding a stuffed animal
hopping off a couch
knocking on a front door
mowing a lawn
opening a car door
playing on a swing set
playing with a tennis ball
reaching for a purse
reading a bible
riding a horse
running from bees
seeing a squirrel run up tree
sitting on a bed
showing affection to a spouse
sitting in a stroller
sitting on the floor
sitting on a porch
slipping on ice
smelling “baby formula”
standing in the backyard
standing in the garage
sleeping
taking out the trash
walking on a beach
walking down a sidewalk
unloading bags from a car
watching TV
Pit bulls are predatory
Predatory attacks occur when the victim did nothing but exist in proximity to the pit bull; they can happen with no provocation and without warning.
http://safetybeforebulldogs.blogspot.ca/2014/04/pit-bull-attacks-with-no-warning-or.html
A study of dog bite victims found that most people bitten by a dog in a public place were not interacting with the dog.
Cornelissen, J. M. R. and H. Hopster (2010). “Dog bites in The Netherlands: A study of victims, 649 injuries, circumstances and aggressors to support evaluation of breed specific legislation.” 650 Veterinary Journal 186(3): 292-298.