My guess is that since the DLC is about the Cabal, and in one of The Taken King missions it mentions that the Cabal are trying to contact their Emperor and summon their whole empire to fight. What I honestly think what happened is that since the Cabal have known about Oryx before us and they found him in our solar system that they called their empire to bring all of their forces in and take him out. But since Oryx is dead now the empire is coming and I'm assuming that the next Cabal to be in command at the time that their arrive will tell them that the guardians are the biggest threat because we took out Oryx. So they're probably going to lead an armada of Cabal ships to come and attack Earth. Edit: That or the Cabal empire will attack Rasputin. Rasputin is able to hack into Mars so I wouldn't doubt it if when the empire arrives that they can get some elite Psions to hack into his system. (And possibly make Rasputin attack the city in some way)