There are rumors of how the Trolliverse is formed from a catalyst
List of Canon Theories
A dominant god declares war on the entire Multiverse he ruled.
It is said that the catalyst was caused when a crazed chief deity began using some universes as throwable objects to fight a large group of nations due to fears that they may overthrow him due to advanced technology. However, he is responsible for the events due to the fact his hubris led him to attack a bunch of nations, therefore leading to the demise of all the inhabitants of the Trolliverse (Save for those nations who migrated to the Finn168719verse, a nearby multiverse).
Wyz'Kaflovyx has awakened from his slumber
It is widely accepted that he has awoke after the Ancient Leaxians failed to provide precious metals to him, causing chaos around the multiverse, forcing much of the Universes to undergo the Big Crunch.
Although it is the least mainstream of all of the theories, one theory proposes that a lower class of gods went to war with each other over Trolliversal domination. The higher-class gods, known as the Precursors to conspiracy theorists, attempted to create a copy of the Trolliverse and to transfer the lower-class gods there to save the Trolliverse's life forms. However, a rogue Precursors cut the tethers of the Space-Time Nexus, creating an implosion in the Trolliverse's center. The result of the implosion was the combination of an alternate reality and numerous universes.