Why does Cyber bullying occurs?

In many regards, it is the ease of use.  Cybe Bullies feel they are protected by the 'anonymity' of being on line.  They forget that they can be identified easily and feel somehow protected by being in their own home. 

The bullies think they are anonymous

Same reasons as classical bullying, except on a computer, it's easier to think that one is bullying a program or a profile, not a real human being.  That happens in real life, too, but it's harder when the person is right in front of you.