To be honest, I think it is because people are social animals (not unlike dogs). We are not meant to be loners, but rather we hang in small groups, striving to establish social order. If you watch packs of wolves or wild dogs, you will see something similar. Members attack the most different (usually the smallest) member in an attempt to prove both their worthiness to the pack, as well as to try and raise their overall status in the group. I think that's what is behind this behavior in humans, at its core.
The good news is that most of us also have a sense of morality that packs of wolves don't have, and most of us know not to act that way. Usually, most people would frown on behavior like that.
Sometimes its jealousy, sometimes its because they fear the unknown, sometimes its because of their own insecurities, and sometimes they're just mean. Its ok to be different, and don't ever let anyone tell you otherwise or bring you down :)
People like to feel good about themselves. One way to feel better about yourself is to feel like your above someone else. Kind of like how in the south during slavery poor whites took pride in being above slaves even though they themselves didn't own any and weren't really that much better well off. Also, sometimes people aren't happy because they don't feel appreciated, loved, or accepted, so they express their pain by making fun of others
I've always wondered the same thing. Maybe it's because any difference poses a threat to the standard that they're trying to conform to. Orator it's because they simply want to make themselves feel more accepted. However, differences are what makes life so strangely beautiful. So dare to be different.