Thanks for the suggestions!

I just want to figure it out enough that it can be there, the event most distant in the past that is dealt with on this planet. It's what happens after it sets the stage that is most important.

I'll likely work on the idea of a massive belt of ice intersecting the planet's orbit long enough for this volume of water to accumulate in the atmosphere, but brief enough to overwhelm the people, killing them all. Muahhaha. Or right, inside voice.