Maybe some herbs or supplements he took were poisonned, maybe the herbs o the supplements he took were toxic on the long term.
Maybe a part of the magnesium sulfate (which i consider toxic considering the elimination crisis it produces on the body) can go into the blood and create deposits (like with calcium or uric acid) maybe in some conditions, one of those deposits can detach itself from the walls of the blood vessels and reach the heart or brain where it can cause a problem.