The Timeline Is Altered
Part of our surprise that “Crisis on Infinite Earths” is coming next season is because The Flash had previously been teasing that it’s due to happen several years down the line. Since the pilot episode, the show’s recurrently featured a newspaper from the future that states Barry Allen goes missing in a “crisis” in 2024. This is also a huge deal in season 5, as Nora West-Allen has come from a future after he vanished.
So, how come the Crisis is now set to happen in 2019? Well, sure, it’s possible that all the heroes jump forward to deal with it – the Legends own a time-traveling spaceship and speedsters can run to whatever time zone they want, after all. However, the more dramatic route to take would be for time to be changed and the event to be brought forward by five years.
It’s also possible that the answer to why this happens has already been teased. We know that Nora’s arrival in the past has changed history – see: Cicada – so what if she also changes the circumstances surrounding the Crisis? The Flash‘s 100th episode revealed that she’s secretly working with Reverse-Flash, after all. Maybe he manipulates events to bring about the Crisis, for his own ends?