when you ask this question it NOT looks like you understand what i saying
there is no "situation" in terms of mechanic/moment abc in dungeon xyz. the situation i talk about is, when dps fail on afixxes (or do not perfectly handle them). this could and will always happen. there is no "point" where this will happen. thats the reason why its unpredictable.
its that easy: nowhere in a raid (besides some specific mythic raid situations) a single person could "not move", react wrong/slowly, etc. and drop down that heavy, like in m+. its way more squishy, and more focused to single chars than "raid drops". or you can look at it like this: m+ feels sometimes, for healing, like if you have 4 tanks. more single spiky targets, than a whole raid drop.
you cant predict if 3 guys drop to 75% over the next few seconds, or if 1 guy drop hard to 20%, two times in a row, while the other 2 guys stale at 100% life.
m+ is squishy. its unpredictable in terms of incoming dmg. so you are 80-90% of time dependend on reactive healing. and thats, sorry to say so, the strength of holy and the weakness of disc.
hopefully now its more clear, what i try to say.