The razing of Earth's surface and the invasion that followed, the grievance driving Carl toward the fuse.
The Collapse is why Carl wants to press The Fuse at all. Weighing the override, he thinks of the legions of bastards who descended on the planet and asks how long it would be before they could do this to someone else, and how many of them he would kill. His anger at the powers that made this happen is, by his own account, bright — so bright.
It is also what makes the memory world sting: coffee at The Homeless Shelter, chips and soda at The Gas Station, a dog asleep on a Christmas morning that no longer exists anywhere but in a recording. In the end the grievance is not enough to make him fire the fuse, and he presses Disconnect Containment instead.
The atrocity fueling Carl's rage, and the reason the memory world exists to be visited.