Emperors are the greatest threats with power and influence. Dragon is a threat in that he directly challenges the power structure of the World Government. The Emperors don't really directly attack the government and are happy to stay in their areas of the New World.

Buggy likely got it from already having territory and influence due to his business as a warlord. Pirates all over were working as mercenaries and flying his flag in order to get around any issues with the marines. This probably led to Buggy's name becoming increasingly well-known throughout the world. He also likely did something after escaping or somehow defeating the marines sent after him. A good theory is that he connected with other warlords and formed his own mini Rox crew.