I have healed nier raid as AST more than 10 times and seen few scenarios play out. Its mainly the issue of lack of communication on which tank gets which.

70% of my pt members are all in south while party tank gets suddenly the north one. Its really hard to keep everyone alive when they are all over the map but even so I always try to run to the other side if tank drops under 50%, I have never let my tank die there.

Sometimes my co-healer is very slow at healing everyone that I need to run up and down to keep people alive otherwise all dps die or tank.