In my experience, the number one cause of failure in Fire over Morgrave is people killing the dretches in bad locations. When they die, they explode, doing massive fire damage in an area.
Other examples of "MOAR DPS" not always helping include:
FoM: Destroying the Risia Ice Orbs the moment they spawn (destroying them removes one of the DoTs in that raid, as well as the fire shield on the boss, but only to those directly beside the orb on destruction, so you want to NOT destroy them so they can be used if needed)
DoV: Killing the hydra heads before they are repositioned into the appropriate ritual circle will cause it to reattach and heal the hydra.
MoD: Killing Lady Vol causes auto-failure of the raid.
Skellies: Killing the Ghost Flame results in the bad ending (only one chest)
Skellies: Killing the Dark Secrets too quickly can result in everyone dying as every time one dies it causes raid wide damage. You want to control the rate you destroy them so the party can heal through the damage (or have everyone in the party with the dreamscape feat and time it for when they auto-explode after 45 seconds each time they spawn)
DWyrm: Auggie summons shadows at specific HP values, and when she is brought too low will heal, but when she heals, she also force-teleports everyone on the shadow side back, so too much DPS means the shadow team cannot finish their objectives as they keep getting teleported away. Auggie also has to die in a specific location, so DPSing when she's not there is counter-productive.
PN: The quartet of bosses have to get hit by the laser within a small window after dying. Overdoing DPS without making sure the laser user is ready and/or the boss/laser aren't in the correct position means they just come back at full, repeatedly.
HoX: DPSing the puppies makes it nearly impossible to complete, as the only way to do significant damage to mommy is by charming the pups with the control stones dropped by the illithids. Killing the pups means waiting for them to respawn to try again, or using one of the very few ways to bypass her immunities (which do so little damage as to be not worth the effort)
Shroud: in phase 2 if a DPS is constantly killing fast moving bosses close to the crystal, it's possible to create a situation where the 4 are never dead at the same time, or dead at the same time long enough to hit the crystal.
CoS: Strahd and the 3 brides have to be dead at the same time in order to interact with the sarcophagus in order to end the raid. DPS who are not paying attention, constantly killing the brides without killing strahd first, often results in all of them healing back to full to the frustration of all (strahd stays down for almost 10 seconds, the brides only for 3 each, so timing of all 4 deaths matters).
LoB: Destroying too many pillars to quickly robs everyone of a second end chest
Reavers Fate: mentioned previously, killing the stormreaver before he lights up all the ceiling balls and someone hits the western lever bricks the raid.
etc