Actually, you've given your answer right there without realising. The Cabal had been fighting Oryx and running from him for a very long time. Oryx and the Hive have been fighting the Vex. What a lot of people don't realise is that upon taking down Oryx, we've freed up the Cabal to mount a full assault against us like nothing we've ever seen before. Do you recall the distress signal they sent back to the Empire? Killing Oryx was the message they needed to send their full forces. We also still don't full understand the goals of the Vex, either.