Here's all I know. We're pretty much living proof that the multiverse theory isn't just a theory. We know from the flip side and gathering Infinity Stones, that timelines can be manipulated or branch off. The doc with the world's worst bedside manner and most loyal outerwear ever? He popped in when things flip-flopped, on day one, and we splintered off in two parallel directions.
There was an instability that started when he messed with time and space using the Time Stone. Some incursion on Earth from another dimension trying to eat our planet's face off (and everyone in it). Demon dimension or whatever. It kind of started to split things off, like a crack forming in glass, but no one was aware of it really until later on. Before Thanos, and after things supposedly went sideways with Asgard. For all we know, Asgard going boom could've caused it. I'm not sure of the dimensional density of their home-world, but they seemed to be connected by a lot of wormholes to everywhere else that Asgard was protecting. If that's what their Bifrost was....
Now I'm theorizing out loud. Oops.
Anyway. Dumbledore's said it's more of a metaphysical thing than a physics thing. Like the universe trying to set things right again by drawing certain important people into this timeline, to protect it. That's his best guess and he's vague about giving answers. That's all we've got to go on about what dragged us in here, for however long we're able to stay. And there's no rhyme or reason why people leave all of a sudden, either.
Answer so far: Big fat shrugsville. But I'm open to other theories.