Once upon a time, there were two opposing gangs in Los Santos. One day, they had a great war. And the winning gang is the one whom the player rules over. Eventhough he is the leader, he still considers his members as friends.
After three years, the leader of the gang which lost at the war came back to Los Santos after his expulsion to California. He is sending his underlings to attack several bases of the player's gang.
The name of the leader of the old enemy gang is BB, which can be taken as just an abbreviation. Considering how long he had waited for this moment, he must have brought so many underlings with him to Los Santos.