Logan doesn't depict the most optimistic future for mutantkind. For X-Men fans who thought all was hunky dory after the rebooted, death-free timeline of Days of Future Past, the new film reveals that that happiness is short-lived (assuming the two films take place within the same universe, which they may not). Mutants are again on the run in Logan, and the X-Men have disbanded, and the main cause of this is the so-called Westchester Incident. But just what is the Westchester Incident in Logan? Spoilers ahead!
One of the key (and most creative) plot points of Logan has to do with Professor X. You've probably gleaned from the trailers that a) Charles Xavier is super old in the movie, and b) the X-Men are no longer around. Those are both truths, and they are also related. See, in Logan, Professor X is actually starting to suffer from the type of mind degradation that typically happens to 90-year-olds. Whether it's dementia or Alzheimer's or something else isn't the point — what is the point is what that means for the people who come in contact with Xavier. The powers of his mind are nearly limitless, and when those powers are out of his control, bad things can happen. And that brings me to the Westchester Incident.
The Charles Xavier School For Gifted Youngsters, AKA the X-Men's headquarters, is located in Westchester, New York, and the Westchester Incident refers to something that happened there some time before the events of Logan. Specifically, during one of Professor X's mental breakdown episodes, several mutants were killed and many more were left paralyzed. The film doesn't specify who they were, but it's implied that both young students and some X-Men perished, and the incident clearly had a profound effect on Logan, Professor X, and the X-Men.
Now, the main question many fans are left with is this: Will we ever get a chance to see the Westchester Incident on screen? The movie definitely leaves the door open for a prequel, but maybe a more important question to ask is whether fans would even want to see that film? Since they already know that it's going to end tragically, would they line up around the block to see Professor X kill children and fan-favorite heroes alike?
My guess is they wouldn't, but given the multiple directions in which the X-Men Cinematic Universe is currently branching out (Legion, Deadpool, New Mutants) in the hopes of finding their new identity in the post-Hugh Jackman landscape, I wouldn't be too surprised if they do end up making X-Men: The Westchester Incident.