In the mid-1990's, Xavier and Gabrielle Haller's son David grew tired of Magneto constantly fighting against his father. Using his powers, David traveled back in time to when Xavier and Erik Lehnsherr first met.
David intended on killing Erik, defeating him before he can become the evil Magneto. But the young Xavier sacrificed himself to save his new friend and took the brunt of David's attack. Xavier died, causing a paradox.
With Xavier dead, David would never have been born and Xavier would have never formed the X-Men, which means, if David never existed, he couldn't travel back in time to kill his father. Since the paradox was created, space and time itself began to fall apart.