Injuries from black bears occur (off the top of my head, in no particular order) because:
A) the victim sees the bear but fails to recognize the behavioral danger signs of a predatory bear,
B) the victim is preconditioned to believe that black bears aren't dangerous,
C) the attack occurs rapidly in heavy cover, or in the dark, or for some other reason the victim has no time to react,
D) the victim doesn't drop food he's carrying, or is feeding the bear, or poor garbage management is practiced,
E) the victim runs from the bear,
F) the victim plays dead, or takes a submissive posture,
G) the victim shoots, but only wounds the bear without stopping it's forward progression,
H) the victim kills a protective female, and is subsequently attacked by second year cubs,
I) the victim surprises the bear . . . who was sleeping, or otherwise in condition white,
J) the victim is simply unaware of the bear,
K) the area is not recognized as bear country, particularly suburbs and agricultural areas,
L) the victim was teasing the bear.