First up: spoilers for Logan if you’ve not seen it yet. This isn’t hugely ruinous but it will discuss the Westchester incident and how it relates to Professor X. In Logan the Prof is suffering from an unnamed degenerative brain disorder causing seizures that paralyse everyone around him. Several times during the film there are references to a previous incident at Westchester, with the implication that, at least once, this condition caused something bad to happen. (And it’s worth pointing out that Salem, Westchester, is where Xavier’s School for the Gifted is/was located).
Source: Logan’s director finally explains what the Westchester incident was about | GamesRadar+