This is just my own vision of how the Cabal attack the Tower with such devastation and trap the Traveler in the beginning of Destiny 2. This is just a theory/daydream, but were I the one writing the story, this is how I would've written it: I believe the Cabal likely use a diversionary tactic to draw the attention of the Vanguard and the bulk of Guardians away from the City, then use that time to launch a surprise Blitzkrieg on the city. My theory: The Cabal appear out of nowhere and unexpectedly attack the Reef with a large secondary force comprised of nearly all of the known remaining Legions in the system, since both the Queen and the Prince are both missing and thus the Reef is considered weakened. Petra sends a cadre of Queen's Guard as emmisaries to the Tower to request help from the Vanguard to defend the Reef. The Vanguard decides they can send the bulk of their fighting force to help, since the Fallen have left the system by all observations, the Vex have been greatly weakened, and the Hive's primary leadership has been taken out. The system has been comparatively quiet since the defeat of SIVA and Guardians have been getting restless and are itching for something to do, so it seems like the perfect opportunity for them to blow off some steam and repay the Awoken at the same time. During this time while the Awoken and the Guardians are fighting the Cabal at the Reef, the City is on a skeleton crew of lower level Guardians and the usual permanent Tower staff, such as the NPC's we interact with. While the Vanguard leadership is holding a meeting to discuss the battle at the Reef (and possibly the sudden disappearance of Eris Morn), the Red Legion launches a surprise attack on the under-defended City, ousting the remaining Vanguard from the City and using the distraction to place a containment field over the Traveler. Suddenly the Guardians all across the system, Reef included, find themselves cut off from the Traveler and the bulk of their Light as their Ghosts drop to the ground, inactive. They are forced to escape permanent death by taking what is left of their ravaged fleets and running in every direction to various enclaves of safety until they can regroup and find a way to regain their abilities. And as Variks gets into his own ship and prepares to evacuate, he stares out the viewport and watches the last few remaining Guardians flee the Reef. He chuckles morbidly to himself, saying, "And now the Guardians are Fallen, too..."