A while back the late bronze age collapse was totally ascribed to the impact of the 'Sea People' (hence city burners?)- based on a few bits of evidence, namely the record of the Egyptians, from which the general name of the raiders was ascribed. (They actually named a few, unfortunately we don't really know who these people are, although I believe there is quite a lot of evidence that the Egyptians after defeating this military alliance, settled some of the tribes in Palestine, the Philistines being a good candidate for one such tribe.)
There are also some plaintive cries for help from archives from Hittite lords on the coast, saying that the enemy were everywhere and their navy wasn't there to defend them.
It meant that when archaeologists looked at ~1300 BCE sites and found evidence of burning or violence, typically the 'Sea people' were blamed.
However a great deal of work, looking at archaeological evidence and new documents has complicated this picture remarkably. You have to remember that pirating and opportunistic raiding was probably endemic at the time. (Think of Troy - a story for later generations of Greek 'Vikings'! They would also have traded, much like the later Scandinavian Vikings.) Also some of the destruction sites don't really add up to savage attacks by outside people. Some cities only burned their temples and rich districts - was this rather an overthrow of the ruling class by the rest of society?
There was great upheaval and empires did collapse - but the reasons for this are, as tinkerdan states 'we just don't know'. There were very probably a number of factors. Climate was changing at the time and this was likely to cause famine (possibly starting in central Europe and causing peoples to migrate East, before drying conditions hit the Mediterranean world proper later) This would have led to more pressure for raiding and whole movements of people. But also some of the empires were in trouble - the Hittites were undergoing turbulent times with their royal families infighting (again for a number of reasons) - and therefore not being able to respond to external pressures. With central authorities collapsing, the trading networks that had been occurring that had been enriching everyone would have collapsed. I believe at this time, the Hittites even invaded Cyprus - a former ally - and the evidence seems to be that they did so to take control of as much Copper as they could - which they needed for weapons. Cities, particularly in the Levant who operated as entrepots, would have had their livelihood destroyed. So add economic ruin to it all...
In essence the Sea people have had a bad press, being blamed for everything. I'd like to think they were a symptom of the real reasons that caused chaos, not the actual cause (although of course, I should also admit that their actions were not helpful!)