Why do you think the psyker becomes dominated?
Stress caused by them arseholes, making them weak for corruption
and why do the chaos agents get so many subjects?
Because their life is shit and they are promised a better life and become ripe for corruption for this
Nobles don't give a fuck about the well-being of others => others suffer => they become weak => CHAOS LULZ!
They could have prevented it, always, but they 're all so fucking useless, damn.