Short answer: nobody knows.
A scholar by the name of Robert Drews presented a list of forty-seven major cities that were destroyed during this period. Not just the subject of battle or hardship but outright vanquished, never to be inhabited again.
Drews claims that every major settlement between Pylos in Greece and Gaza in the Levant was destroyed and abandoned. Forty-seven were credibly identified as having been destroyed during this period, and the number is probably much higher in actuality. . . .
What could have caused such a massive failure of civilization in such a relatively short period of time? A hundred years is nothing to the vastness of history and humanity had never seen such a fast and decisive collapse before.