There are tons of causes for this, just a couple off the top of my head:
People who spend a majority of their time on the internet (not including work time) tend to have generally poor real life social skills. There are tons of reasons, poor self confidence, anger issues, depression issues, etc etc. But the poor social skills are then amplified by the feeling of anonymity.
There is also "peer pressure" like issues that ties into the ones mentioned above. Many people want to fit in and they tend to follow those who are mean and abusive because they think that it's a symbol of power. So after time they start acting like this as well.
These are the resons I personally see most often, but by no means does this explain even a tiny bit of what we see on the internet. Even I am a bit vulgar and down right mean at times. But I only do that with people with whom I am friends with and our "mean/sexism/d0uchebagerry" is actually our playful rapport.
But Impact is correct, compare to people in FPS shooters like BF3 and COD, MMO's sexists/bullies are kittens.