Bullies are voracious opportunists. They usually select their targets for various reasons. It may be because you’re good at your job, popular with your colleagues, clients, customers; any of the aforementioned could make you a target for a Bully. 

In the end, a Bully fears the exposure of his or her own inadequacy or incompetence, and your presence, competence, and popularity unwittingly agitates those fears.