Pastime Sans controlled the Past, Future Sans controlled the future. Present has to make sure none of these two are being mischievous. Pastime seems to be more unpredictable while the Future is less unpredictable. These 3 AU sanses control the timeline and tend to try to make monsters forget the life they had in the past (Pastime's job) before the resets.
Pastime sees how Present fixes the past more than the future, and is angered by this. Pastime defeats Present and goes to attack Future. What Pastime didn't know was that those two could not touch, or else their souls would fuse. That was exactly what happened. It caused a rupture in the timelines and the two fused, creating Paradox.