I have heard of rogue fish like that. I never heard any theories about why it happens, though. Captivity has a widely strange affect on certain individuals of just about any species. I once had the pleasure of taking care of a rabbit that had become extremely aggressive for no apparent reason. I had to where heavy leather gloves in order to clean the cage because it was guaranteed to attack anything that entered it's cage. It happens the other way around, too. Individuals of aggressive species, even cichlids, can become very peaceful, even personable, with their human caregivers. I've even pet a few from larger species. Oscars being the most common that seemed to actually enjoy it.