The Planar Collapse

Enmilon was not the god's first attempt at creating a world.   It was merely their first (partial) success.  In the three previous attempts, the Planes would collapse in on each other as soon as they were all in place.  Finally, on their fourth attempt, the gods successfully stabilized the planar cosmology, or so they thought.   Weary of causing another collapse, they decided to make the rest of their creations far away from their recent success.  Thus the distinction between the Inner and Outer Planes was created.  Centuries later, when they were putting the final touches on the Outer Planes, it began to happen again.  All the Planes began to collapse in on each other.  The gods desperately tried to stop it.  Even Enmil himself, who had remained only an observer since he created the other gods, took pity and offered his assistance.  With their combined effort, they stopped the collapse, but only after the Inner Planes had already partially overlapped, and were firmly entwined with each other.  The Outer Planes were also much closer to the Inner Planes than before.
The gods debated for decades about whether to start over before noticing that life had continued to flourish, even after such a cataclysmic event.  Much of it had been destroyed, but that was a trivial issue to fix compared to creating an entire new cosmology.  Ultimately, the gods decided accept their creation for what it was and move on to work on its finer details.