People actually do that. I think the motivation is similar to bullies at school but I don't know for sure. Hasn't happened to me for a while but it is one reason why I never use the same nickname on any two forums. Google makes it too easy for the stalker to find you if you do. Has happened to others on this forum.
And yes it is probably illegal. But unless it is life threatening - law enforcement is unlikely to be interested in doing anything but forum moderators might ban the stalker if they agree with the complainer's assessment.