Do YOU know the reason for this?

well you probably are a shy person, maybe they tried to approach you once and they get your lack of interest as a personal affront. they'll never stop. now if you try to talk to them, they'll get worse, because they'll think you're trying to look cool by talking to them. besides, they're totally shitting you and you wave them? they were like 'haha, there's the stupid b*****, look at her ..' and waving you as to a 2-year-old and you respond? sorry but that was stupid and so was answering the door. you're confused and they know that. just try to have the same reaction every time, and that means ignore them. there's nothing you can do, just a bunch of 'gangsta' bulliers. just act like you don't care and you will , eventually. that's what they want, to get you upset or even make you cry. but if they don't try to touch you it's ok, they'll never achieve what they want and you won't get hurt either. try to listen to music while being outside, so you don't hear all the crap they say. take care!