Source: Christopher Rowe, "Plato," The Cambridge Companion to Greek and Roman Philosophy, ed. David Sedley (Cambridge: Cambridge University Press, 2003) 111.
Christopher Rowe refers to Plato's Statesman, in which the character Socrates discusses a view on the cosmos' cycles. In this view, there are "two recurring eras in world history, one golden, belonging to Cronos, and one belonging to Zeus, in which we ourselves live; and these two eras are separated by a shorter period of reversal, when the deity has 'let go of the steering oars of the universe', at the appointed time. In this period, which begins and ends with great destruction, dead bodies come back to life from the earth, and get smaller as time goes by, until they disappear altogether."
There are two explanations for how this disintegration occurs. One is that the cosmos is dynamic and can't stay the same forever. Another is that the universe wants to be free from the restrictions of the deity, so it breaks loose temporarily. In this model, it eventually remembers the deity who organized it, and "it returns to its proper course."
And the universe is a parallel to the human soul, according to Plato. In the same way that people need to subordinate their unbridled passions to reason, so must the universe embrace rationality, notwithstanding its desire to break free.
This reminds me of two things:
First, in the Hebrew Bible, God often has to fight chaos in his attempt to restore justice and order. The ancients realized that the earth has order, as proponents of Intelligent Design like to point out. Yet, it also has a lot of chaos, which can be quite destructive.
Second, it reminds me of an Amish ritual--the one where Amish teenagers take a vacation from their religion to do their own thing. Many come back to the Amish lifestyle. Many don't. Those who do probably get tired of the chaos of modern-day life. The same is true for people who sow their wild oats when they're younger, yet become more responsible as they mature.
I guess Plato believed that the universe needs a vacation from order every now and then. I don't know how often he thought this disintegration took place.