The Flash's daughter Nora could have accidentally set up the "Crisis on Infinite Earths" event when she destroyed Cicada's dagger. Nora West-Allen, Barry's daughter from the future, was a major recurring character in The Flash season 5. She'd traveled back in time in order to get to know her father - and she vaguely dreamed of somehow rewriting history, preventing his disappearance in the Crisis that was destined to happen in 2024.
The Flash season 5 came to a dramatic close when Nora realized she'd been manipulated by Eobard Thawne, the Reverse-Flash. Thawne had tricked Nora into destroying Cicada's dagger, a shard of metal suffused with Dark Matter that nullified the abilities of any nearby metahumans. Destroying the dagger changed the timeline - and erased Nora. Surprisingly, it also moved the Crisis forward in time to 2019. In The Flash season 6 premiere, the Monitor told Flash that he will now die on December 10, 2019. That neatly explains why Nora has been wiped from the timeline; because Barry will perish long before Iris becomes pregnant.
But why did the destruction of Cicada's dagger accelerate the Crisis? The mystery has only deepened as a result of The Flash season 6 and Arrow season 8. The Flash season 6, episode 2 revealed that the Crisis has been going on for over a year now. Barry Allen traveled to Earth-3 to talk to fellow super-speedster Jay Garrick, who revealed that he'd been monitoring a Multiversal crisis for the last year. He'd been detecting mysterious antimatter signatures, and had even been able to run some sort of algorithm that told him the entire Multiverse was in danger. One of these antimatter surges was encountered by Oliver Queen at the end of the Arrow season 8 premiere, when Earth-2 was destroyed. That means worlds have been dying long before Nora destroyed Cicada's dagger - so how has the Crisis moved forward in time?
The most logical explanation is that, while the antimatter wave may be consuming the Multiverse, it is actually drawn to a world by the actions of an unknown metahuman. In the original timeline, Cicada's dagger was used as a tool to keep dangerous metahumans imprisoned and depowered; it must have held this mystery being in check, preventing them from drawing the Crisis to Earth-1 - until 2024, when something happened that meant they escaped the dagger's inhibiting presence. But the dagger has now been destroyed, which means the timeline will run differently. This mystery metahuman can't be inhibited now, and as a result they summon the Crisis in 2019 instead. The most likely candidate is the Arrowverse's version of Psycho-Pirate, an Arkham Asylum inmate. "Don't worry, Doctor," he assured his fellow inmate Doctor Deegan. "Everything is as it should be. The stage is set. Worlds will live. Worlds will die. And the universe will never be the same."
Nora may have effectively erased herself from the timeline, but she could well have unwittingly saved billions of lives. The Crisis was already consuming realities a year ago, long before she changed the timeline, and in the original timeline the Multiversal catastrophe continued for six years before Barry Allen's sacrifice brought it to an end. As a result of this change, the Crisis will happen in 2019, and hopefully the heroes will still be able to bring it to an end. If so, countless Earths will be saved from destruction by the premature end of the Crisis on Infinite Earths.