Some social scientists believe it was caused by years of drought or bad storms. Some think it was earthquakes. Many believe it was due to invasions (including mysterious attacks by a group that Ramses III called the Sea People), or mass migrations. There are theories of systemic problems. Robert Drews argues that improvements to the use of infantry, as well as the invention of more deadly swords and spears, made the old chariot-based system of warfare vulnerable.
We know that, in that brief time span, many cities and palaces were sacked. There is evidence of mass slaughter, looting, and fires that destroyed entire cities. In most cases the cities were not re-inhabited. It is odd that raiders would so frequently move on after sacking a city; or if they did, that local people wouldn't rebuild. That is also the problem with earthquakes or droughts as explanations: why then are the cities permanently abandoned? In some cases there were hidden hoards of jewels left by occupants before they either fled or were killed, hoards that were never recovered until archeologists discovered them.
The Egyptians managed to survive the Catastrophe, but the pharaonic system was crippled permanently. A few scattered cities survived, as did parts of Syria. Other than that, the entire eastern Mediterranean succumbed to whatever it was.
The really jaw-dropping aspect of the collapse was how rapid it was. During as little as 20 years there was a huge decrease in population over the whole area, and an even greater decrease in the number of people living in cities, towns and even villages. Ruling classes were wiped out, and no central authority replaced them. Written language vanished in most places. Artistic quality diminished. People started living in mud huts. The area entered a dark age that lasted for a thousand years in some parts, and 400 years in Greece.
Homer lived at the end of the dark ages, and he wrote about events set at the beginning of the dark ages. The sack of Troy in the Iliad describes the destruction of one civilization, and the threat to Odysseus' palace in the Odyssey describe the problems throughout the area resulting from having kings and armies away at Troy for nearly ten years. Homer would not, of course, have been thinking in terms of the destruction of the Mycenaean palatial civilization or the end of the Bronze Age. Nevertheless, he provides a plausible explanation for why civilization suddenly vanished hundreds of years before his birth.
In the Iliad, Homer describes 150,000 Greek troops at Troy. In addition to the Greek forces, the Trojans called in tens of thousands of allies to help defeat the invaders. At the time of the Iliad there were roughly 40 million people on the entire planet, so the armies alone amounted to nearly 1% of the world's population, and a hugely higher percentage of the population of the Mediterranean. In fact, it might be argued that Homer's numbers are impossible because they represent more fighting-age men than existed in the area at that time. The forces were deadlocked at Troy for nearly ten years.
The invading Greeks included scores of kings. Their absences from home caused all kinds of instability, as did the absences of the armies. In the Odyssey, Homer describes Odysseus's wife Penelope besieged by suitors trying to take her missing husband's place. The Odyssey (and other texts) describes the fate of Agamemnon, leader of all the Greek nations, whose wife took up with another man and killed Agamemnon when he finally returned home. Legends abound of similar fates among other kings who were bogged down in Troy.
The number of troops at Troy suggest that every adult male was away from home. That would include authority figures, experts, shop-owners, scribes, teachers, artisans, bureaucrats. In the society of the day, it is unlikely that women would have been able to step up and take over those roles.
As to the utter destruction and non-rehabitation of cities, in The Fall of Troy Quintus says that the invading Greeks completely destroyed the city and then immediately sailed away. In the Odyssey, Homer says that the Greeks "utterly destroyed the Trojans' city."
If the decade-long siege of Troy explains the regional instability that destroyed every civilization involved in it (and some beyond), then what caused the war in the first place? At the center of Homer's explanation is Helen. The war happened because Agamemnon wanted to get back his sister-in-law, who had run off with the Trojan prince, Paris.
You can argue that Helen isn't to blame: Eros hit her with a love dart, Zeus masterminded the whole thing to bring down Achilles, etc etc. More plausibly, there is some reason to think that Agamemnon wanted to continue his territorial expansion and used Helen as a pretext, although there are also indications that he was bound by oath to get her back, and in the end he didn't gain anything by the war. In any event, during the ten years of the siege Helen stayed with her new husband Paris and did not attempt to go back to Menelaus, which she could easily have done.
Helen isn't the only beautiful woman in the Iliad, or the only captive woman, or the only beautiful captive woman that men fight over. Apollo attacks the Greek forces because Agamemnon won't give Chryseis back; and Achilles refuses to fight because Agamemnon won't give Briseis back. Yet none of the other women are the cause of anything major.
When the siege is over and Troy is destroyed, Helen returns to Menelaus and goes back to her old life as Queen of Sparta, where Homer has Menelaus (in the Odyssey) describing her placidly as "my dear wife". The only individual in Homer who blames Helen for anything is Achilles - but that's not insignificant, as Achilles is the main character in the book.
Of course, poets aren't historical sources, but you don't have to believe in the existence of the Muses to see that poets sometimes tap some deep truths, or provide glimpses that lead us to insight. (Plato's shadows on the wall come to mind.) In the central causality of Helen, the truth is tantalizingly enigmatic. As Yeats wrote when describing the conception of Helen (the rape of Leda by Zeus disguised as a swan):
Leda and the Swan
A sudden blow:
The great wings beating still
Above the staggering girl, her thighs caressed
By the dark webs, her nape caught in the bill,
He holds her helpless breast upon his breast.
How can those terrified vague fingers push
The feathered glory from her loosening thighs?
And how can body, laid in that white rush,
But feel the strange heart beating where it lies?
A shudder in the loins engenders there
The broken wall, the burning roof and tower
And Agamemnon dead.
Being so caught up,
So mastered by the brute blood of the air,
Did she put on his knowledge with his power
Before the indifferent beak could let her drop?