Either they have bad manners, they're having a bad day (or longer period of time) and their frustration comes out as rude behavior, or maybe they feel offended (I certainly have a hard time not being rude to my housemate when he behaves rude towards me such as cursing at me).
People are rude because they don't have atiquets, they are brought up like that. No manners. They are rude because some times things go opposite way then what they think. They do due to frustrations too.