How did you look at them? Directly in the eye, in a focused /direct manner? I can see how many guys can assume, if girls are giving them a direct, intense stare while observing him walking towards you, that you are thinking "Umm...ok, what is he doing? Can I help you?" Especially if there are more than one of you.
They feel like they are being put in the spotlight as they walk towards you, as if they were going before the supreme court. While its bad to interpret things like this, girls have been known to be *****es in situations such as these.
...
The biggest piece of advice I would give you Milady, Sugarless, BoricuaDelight, and the rest of you, is that many people, some women included, but mostly men, interpret most of their social sitations (especially with the opposite sex), in terms of past experiences and mental labeling. For example, they might meet a girl who is wealthy at a party. Having met a few wealthy girls in the past who were very concieded, this girl is also subconsciously assumed to be a spoiled *****. Or perhaps a few girls in the past had acted interested in him and came up and started talking to him, turned out to be complete teases, so he assumes when girls come up and instigate conversations with him, they are at best looking for a O.N.S. and shouldnt be bothered with. It also can go to the extreme of "all girls are just *****es." Some people do this labeling only slightly subconsciously, some to the complete extreme.
So I would say the reason these guys might act shy, hostile, or just weird is not because of anything that YOU directly did or didnt do, or that there is anything WRONG with you, but rather his past experiences that he associates you with. On the other hand, he may put you on this giant pedastal in his mind, of which he thinks you are far superior to him. Maybe b/c of your good looks, or maybe something else he doesn't have. As a result, he will either be shy and scared (which you might misenterpret as 'not interested'), or he might feel threatened and thus attack or be mean to you so he doesnt feel so small anymore. Most guys overanalyze situations with girls, especially if they are interested, to such an extent that they cannot relate anymore, because they have reduced the other person to a bundle of thoughts, labels and judgements. This is why people of the opposite sex seem so mysterious and hard to understand sometimes.
Just from my experiences anyways....
So I would say just be yourself and dont always get wrapped up in the drama of GUYS and trying to figure out what they do and why they do it, and try not to take it personally, because its usually not meant to be. Most of the time its so complicated that noone can understand it but them, and also because then you are playing their game of complete over-analyzation and many guys are not even worth the energy