why do cats sometimes attack people even if they are doing nothing wrong ?

Question: why do cats sometimes attack people even if they are doing nothing wrong ?
- Keywords:
  aggression,
  attack,
  cat
Asked by jessc to Kevin, Charlotte, Jo, Louise, Valeria on 18 Jun 2012. This question was also asked by michelletherezien, rubberducketc, cerysklewzyc.
- Kevin Mahon answered on 18 Jun 2012:
  
  That really depends! Most cats certainly don’t attack completely unprovoked – obviously cats with mental problems of psychological disorders can and do. In general cats get freaked out by sudden movements, noises and unknown sights/sounds. This varies greatly on the tempermant of the cat themselves so I can’t give a blanket statement! Some cats don’t get along with humans because of their upbringing, again there are many reasons for this!

Question: why do cats sometimes attack people even if they are doing nothing wrong ?