The Universal Incursions began seven years after Earth-616's Doctor Doom set out on a quest to murder every version of Molecule Man in existence in the hopes of stopping a heinous plot by The Beyonders, who sought to destroy all of creation by using Molecule Man as a pan-dimensional reality-annihilating bomb. After the first Incursion occurred, the timeline of the multiverse began to contract, meaning everything would die slightly sooner than intended.
Because of this contraction, the barriers between realities break down completely and at least two Earths along with their respective realities fall into a void that was created when Doctor Doom botched one of his assassination attempts and accidentally "detonated" a Molecule Man, leading to the destruction of that Molecule Man's reality. As this happens, an Incursion Wall goes up which links a location that exists on both worlds together, acting as a bridge between the two universes, which draws the Earths towards each other.
While this process falls into place, there is a short period of harmonic alignment between the two Earths, allowing both to exist in the same space for a span of eight hours before a collision between the two Earths occur, which results in the annihilation of their respective universes. The only known method of stopping an Incursion from occurring is to destroy one or both of the Earths that have been drawn into the Incursion Wall before they can collide, doing so will cut off the connection that has been established between the two realities.
The concept of a Universal Incursion was created by writer Jonathan Hickman and first appeared in the story "Momento Mori" in New Avengers #1 (Feb. 2013).