It has nothing to do with reality.
While neither of my parents drank or did drugs, my mother was frequently yelling at all of us for random, stupid, pointless things. I learned later there in her case, there were two main causes:
My father was hardly ever home (he has problems saying "No" to people, so would work extra at work, but would never tell my mother he'd be home late) so she took her frustration out on us, when really she was frustrated that my father wasn't around to help her with things he'd committed to do.
My mother's mother was extremely old fashioned, objected to my mother going to school, refused to pay for any of her college (so my mother worked full time and went to school part time to get her degree), and she resented that we her children had "better opportunities" because she was helping pay for our college.
Neither of these were my fault. She decided to marry my father and not put her foot down (or call to make sure he was still coming home). She decided we were all going to college and she was helping to pay for it. Both were her decisions, but we were the targets of her frustration.
It doesn't really matter why they do what they do, because it's their problem, and their problem has no bearing in your reality.