The Last City almost had to burn as a way to get Guardians out of the Tower for a sequel. We already know that Destiny will force players to leave pretty much all their gear behind, so it makes sense the game would facilitate this from a lore perspective by destroying the Tower and the vaults it contains. It’s clear Bungie wants Destiny 2 to be a fresh start in a lot of ways, and destroying the Tower/City seems like a way to make that concept a reality.
As for who is attacking? There was that bizarre leak a while back where a line of Destiny toys implied that the Cabal had banded together to attack the city, and it seems possible that could be what’s happening here. I’m inclined to believe it because after the vanilla game and four DLCs, the Cabal have still not really gotten their due in the game.