Why do guys ignore girls once the girl notices them?

Nothing at all unless your being to pushy for his taste.... if your really not then he might just be scared now that you finally like him he may not know where to go from here and he may be intimidated by you.... Or he could have really liked you last year when you did not notice him and he may have tried so hard to get you to like him then gave up maybe even feeling a little discouraged that you did not think twice so when you did finally start to notice he may had already moved on....

Try seriously talking to him if he still ignores you without anything he could also be extremely immature or just be giving you a taste of your own medicine but at least try let him know how you feel without sounding false or desperate and be completely cool about whatever response you get and keep an open mind to be friends even if he does not feel the same.

We guys like attention and girls do the same usually, wanting more persistence from the opposite sex. Its like reverse psychology. You need to play the game. Find the rules and have some close friends help you

People always want what they can't have. He might have only liked you because he knew you hated him, but now that he knows you like him too he is either uninterested, or if he is inexperienced with girls he could be scared that one actually likes him.