The cops are taught to look at they way people dress and behave, if you are a hoody wearing pants down around your knees kind of guy then you will be looked at, same thing if you look punk, emo, goth, metalhead, are you from a poorer area wearing top dollar clothes, they look at stereo typical types of fashion, also your personal apperance has a lot to do with it, if you have bags under your eyes or look half asleep, have bad skin you will be looked at.
This is their job, this is how they work, when the cops are hassling you they do it for a reason, they wait for your reaction, if you get aggro or your facial apperance gets dark then they will push.
Simple psycology is simple.