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No you misunderstand the timeline of events. LITERALLY right after you take out the weapon aboard the Dreadnought, Your actions also cause a disabling of the Dreadnaught's shields. After the complete massacre of the taken attack on Phobos the Cabal kept a keen eye on the status of the Dreadnaught. Seeing the shields disabled the cabal immediately commence their plan of turning their ship into a beach head on the Dreadnaught. So when you see the cabal ship crashed for the first time. It LITERALLY just happened. Question answered.