Universe Bonding is a type of Dimensional Decay.
A Universe Bonding is exactly like a Universal Overlap, but on a grander scale. When an overlap reaches more than 10% of the universes in question, it becomes a bonding. These universes are basically bonded together and become the same dimension.
This makes the dimension much larger than it was originally intended to hold, and forces the universes to overlap even more. The bounds of the dimension will accelerate any Dimensional Decay the previous dimensions had until all the universes in question are completing bonded (or overlapped).
A Universe Bonding is a fairly cataclysmic event. Many planets and stars are destroyed, and the dimension becomes much more dense with planets and stars. The actual event of the bonding is not the only damage that happens, however. As time passes, the matter must get used to the new matter that they must now live with and this results in some star systems colliding with each other when they have conflicting rotations around the universe. This is part of the reason why galaxies collide with each other -- they keep following their old rotation and will eventually collide into another galaxy that happens to share its same path.