I think it's an unforunate side effect of our human nature, which happens to manifest itself the ugliest in the ultra-hormonal social bubble that is high school.
Everybody (and I mean EVERYBODY) thinks that they're abnormal or weird in some way. Who among us can honestly say they've never had a "Jesus christ, what's wrong with me?" moment? Talk in-depth to enough people and you'll realize this is true. So for many people, a band-aid for this insecurity is lashing out at the person who is strange in a more conspicuous way. The gay kid, the fat kid, the nerdy kid...easy targets because it's much harder for them to hide their idiosyncrasies.
The secret, I think, is to learn to embrace that which makes you "weird". It's a difficult thing to do, but it's one of the few tangible keys to happiness.