Operationalised as 1billion or more people dying in a 365 day range that ends before 2100, conditional on there not being any AGI created before 2100

Yes, but with the causation going in the other direction than you sound like (i.e. the most likely reason we would fail to make an AGI before 2100 is if a nuclear war/major pandemic/Yellowstone eruption/whatever stops us from making one)

Let's jaw-jaw what that could be. Asteroid or rogue planet of course. Would one see a star flung by that super massive BH more than 75 years in? Supervolcano not sure would be that quick. What kind of mass or radiation emission bursts from nearby space? Non AI directed weapons extant or to be invented. Microbials, wild probably not that quick. Designed maybe, particularly mirror ones.

@JussiVilleHeiskanen Nuclear weapons, most obviously. Anything natural would have to be priced much lower.

That's a fitting name to bet yes on this question.