How come when people yell at me?

I'm a nice person BUT i have a firey side as well. The fire side has to be aroused by something like someone being overtly rude to me or trying to belittle my rights as a human(TRYING to pick on me, cos I used to get picked on when I was little but I got into a fight with a girl who came up to me and hit me in my face and i beat her as* senseless and people were shocked because i'm quiet and nice and since that ,I stick up for myself and don't take people's crap if I don't have to but stupid things I ignore, it's just when someone comes at me DIRECTLY tryna threaten my livelyhood/well being, I still have sense and remain calm but if they keep on , well yea lol) I notice that when my mom an ddad tell me to do things ,they HOLLER and YELL at me and it seriously pisses me off but if they had just asked me nicely, i would have done it. I don't need to be hollered at , all they have to do is ask me. When they holler at me, it makes me not do it but if they just asked me nicely, i would have. Why is that ?