The Principle of Celestial Cohesion

Historical Observations

Before the first mortal races were created, celestial beings had free passage back and forth between the Celestial Planes and the Material Realm. Over thousands of years, however, the boundary between the Celestial and Material planes became more impermeable, eventually culminating in The Celestial Exclusion, when celestial creatures were forever barred from entering the Material Realm on their own power. 
But before that day arrived, there were almost three thousand years where celestial beings could enter the Material Realm at certain times and places and always for a limited amount of time. For a long time, the planar boundary became permeable only four times a year: the Vernal equinox, Summer solstice, Autumnal equinox, and Winter solstice. During these times, celestials could enter only at certain locations, usually marked on the Material Realm by some unique or magical natural feature. The gods could wander wherever they pleased on the Material Realm and could bring lesser celestials with them, but no matter where they were, they all would be inexorably pulled back to the Celestial Realm on sunrise of the next day.
These rules applied equally to gods, devils, demons, and angels and even a being who becomes divine while on the Material Realm (for instance, by killing a god) was subject to the same rules.

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Early Theories

Before the rigorous application of scholarly examination, the prevalent theory of how this phenomenon worked was that the gods had made a binding agreement among themselves to give their creations autonomy. However, this theory does not stand up to scrutiny since there are many gods who break agreements with other gods, almost as often as they break agreements with mortals. In addition, it does not explain why demons and angels are bound by the same rules even when they have not pledged their loyalty to a particular god. Others have theorized that there are other gods older and more powerful than the gods we know who impose these rules in order to keep the lesser gods' power in check. But there is no support for this theory other than the ramblings of madmen and fanatics.

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Modern Theory

Scholars in Faerun have studied the planar system's rules regarding divine beings and observed that the Celestial Realm has a particular type of energy that permeates the plane but is not present on the Material Plane. This Celestial energy would invade the Material Realm when the planar boundary became permeable. Divine beings, who are made almost entirely of Celestial energy, could likewise enter the Material Realm. However, when the holes in the planar boundary began to close, the celestial energy was pulled through. Because celestial energy is cohesive, the movement of the background energy "sticks" to divine beings and they are pulled back to the Celestial Realm.

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Changes in the Planar Boundary

Over the centuries, the planar boundary has stopped becoming permeable and no longer allows divine beings into our realm. There is some speculation about the disastrous effects of a divine being existing on the Material Realm now. The celestial energy in the divine being would naturally want to cohere with the celestial energy in the Celestial Realm. The tension would potentially shred the planar boundary, scattering both celestial energy and the astral energy that makes up the planar boundary into the Material Realm. Divine beings would be able to enter the Material Realm at will and the clash of energies could start to mutate the natural denizens of the Material Realm.
But of course, that is all highly unlikely.

Type

Metaphysical, Astral