Perhaps these aren't meltdown scenarios, but you left off some of my personal favorites:

Alien invasion. Either short term (ie: "I regret to inform you... your planet has been scheduled for demolition. Have a nice day.") or long term (ie: failed political relationship with alien beings leading to war).

AI robotic revolution. With humanity's instinctive insatiable greed, they will repeat the history of slavery, but this time, with captives of immense ability.

My third favorite is a tie between overpopulation and pandemic, so I chose pandemic. I'd much rather see the "oh high and mighty human race" be crushed not by its own weight, but by simple multicellular organisims invisible to the naked eye... and by the foolish human politics which would prevent a cure from becoming available soon enough (up for a game of Deus Ex anyone?)

Another meltdown scenario, not in my list of favorites, but has almost happened enough times in our history:

Accident - Oops, mismanaged the nuclear reactor and started a chain reaction. Oops, left the outside air vent open while working with that 1918 flu virus. Oops, pushed a little to hard while cleaning the cheese stake stain off The Big Red Button. Bye bye world.