What do you think these bullies problems were?

ive had a few major bullies in my life but one thing i never understood was why they "attacked" me in the first place. they were all girls by the way and im a guy.one of them was in elementary school and a year older than me and she was a girl, so i didnt know her at all. then all of a sudden she would always say really mean things when we were at recess etc. and she was friends with my older sister and i remember one day at a party she was at with her she said your brother is so stupid and went on about it. im in highschool now and til this day i just never knew why she had a problem with me, she always gave me a hard time for about a year. and then the next year this other girl did the same thing because her friend liked me and i didnt like her back. it just confuses me because how would you feel if someone you totally idnt know just bullied you for so long and you just never knew what there problem was?? they were both mexican girls and not being racist but i go to a highschool where there is alot of mexican people there and alot of the girls there remind me of the ones that gave me a hard time. the two girls that did that go to the same school now and even when i see them in the hallways they give me bad looks. so they havent changed. its really bad to because since they are girls and not guys its nothing physical its purely verbal.