Well Uldren was wandering the Solar system, going back and forth between planets and hiding from guardians. What we last knew of him before AoT he was found by a ton of his crows. Either he or his Crows found out about the major Cabal assault by snooping around on Mars. Uldren has also taken over the House of Kings from what we know in some of the new Grimore. He managed to get his chance to kill the Kell (or take a place as a higher up in the house) because I assume he knew how they operated thanks to working around the House of Wolves and Variks for so long and seeing the Kings would probably be desperate by now to get a better foothold against their foes. So basically one of Uldrens crows found out about the assault and alerted him. Uldren being the Kell (or at least higher up but we will say Kell for now) of the Kings, the House that gives orders to the devils and what not, probably ordered the Fallen to go into hiding and wait for the Cabal to do their thing. After that there are many possibilities. Either we or the Cabal take the city. If we manage to defend it Uldren could either aid us or raid us. Same thing could happen if the Cabal take the city. He helps us take it back or he takes it for himself. There's also plenty of room for betrayal and whatnot as well.