Why does my mom get mad at the littlest things?

Whenever I take showers there are usually towels on the side and they hang there. My mom puts them there, I forget to move them and then she's all like "YOU HATE ME HUH? YOU NEVER LISTEN TO ME AND YOU GET ME ANGRY ALL THE FCKING(excuse my language) TIME" She gets mad even there is a little dot on the towels. She can't even stand my little sister. She's only ten and still growing up, and she recently gained a lot of weight and I tell she's skinny so she won't all like" oh my god! I am so fat!" My mom tells her that she's really fat and she's fatter than anyone in the world. My sister isn't exactly fat, I mean she's still growing up but there is some chub. My mom will also tell me that my pants are too tight. Yesterday when we went shopping she was giving me dirty looks and told me aren't your pants extremely tight? They aren't even close to tight anyway. She thinks highly of herself and doesn't let me have 1 second of freedom. My question is why does she do this to me?