Bill Zebub
“It’s probably Matt Mercer’s fault.”
MMOs have gotten a lot of crap over the years from tabletop players, but I think boss monster design there is head and shoulders above where it is in 5E.
It's an expectation in World of Warcraft, for instance, for big raid bosses to have multiple health-related triggers for all sorts of changes to an encounter, from armies of spirits stampeding across the battlefield to powerful attacks to the floor being smashed and everyone dropping down to a new battlefield on the floor below. (This isn't exclusive to WoW, of course. I believe it was pioneered in EverQuest's ring events back in 2000.)
YOU ARE NOT PREPARED!