Well after this DLC we get rid of Oryx .. Cabal came to our system because they were running from Oryx .. So without Oryx the Cabal are free to attack us .. Maybe they have a grudge on us because we killed Crota and made Oryx come to our system and convert to Taken most of the Cabal .. Everything is a chain of events in Destiny so far so i guess events on TTK trigger the Cabal revenge on us..