The witches showed up at an opportune time.
It's hard to suss out just yet why the witches came to Outpost 3 at this specific time. Perhaps the deaths of their "sisters" spurred them, or perhaps they were responding to something that we're not yet aware of. Either way, their arrival at this specific moment means we're about to get some answers.
First of all, are the witches in charge of the sanctuary outpost that Michael frequently mentions? We don't know what this outpost contains other than better food and a more "impregnable" location. It's possible the witches are in charge of this outpost, and that's why it's so protected. Or maybe they've been hiding out in their own private quarters, waiting for the perfect moment to descend on Outpost 3 and have their way with Michael.
Either way, they entered the events of Apocalypse at the most opportune time. With most of the survivors now dead, and a select few women revived, we can piece together what this apocalypse means, how it might be resolved, and what the future of the world looks like under the guidance of powerful women defying male power.
But maybe the apocalypse is just a vision.
One popular theory is that the apocalypse we've been privy to is just a vision Cordelia -- as Supreme -- receives, and that every episode until this point is a warning sign of what could happen if the Coven women don't prevent it. According to showrunner Ryan Murphy, things start getting wacky around Episode 5, so it'll be around that time that the reality of Apocalypse starts to crystallize. Is anything real? Is everything fake? One thing is for sure: The coven will play a big role this year, as will the cast of Murder House beyond just Michael Langdon.
But how will all of these things come together? For now, it's anyone's guess. But this being American Horror Story and all, it'll likely be something no one will figure out on their own.