In order to break the connection between the universes, the invading universes just needed to start moving away. But doing it would not be possible if one of the connected universe was in the process of merging together.
Even through it was possible to forcefully create the invasion pathway, it was actually impossible to start a forceful merging. And merging was a very dangerous thing to do with a lot of uncertain outcomes. So there were almost no universes who would be willing to agree to this.
But the will of the universe being attacked had gone mad, so it actually did not care at all about the troubles associated with the process. So it lowered all it's defenses and immediately started the merging process.
Obviously the other three universes immediately tried to stop it and get away. And they would most probably be successful in getting away too. But this whole process was going to take some time. About a year or two.
