It's not really known what were the Sea Peoples that entered the eastern Mediterranean Basin, or where they came from altough it's highly suspected they were a mix-up of various peoples from Central, Central Southern Bronze Age Europe, maybe due to an increased climatic stress and ensuing infighting as evidence of large battles have been recently found.

It's also probable that a good part of Sea Peoples that entered Anatolia, Syria and Egypt included groups from Mycenean Greece.
Some peoples listed by egyptians could be tied up to Greece :

Denyen / Danuna -> Danaoi
Tanaju -> Danaoi (Tanaja is listed on a pedestral on the Mortuary Temple of Amenophis III as a land beyond Keftiu, Crete)
Ekwesh -> Achaioi ( related to the possible Hittite name of Acheeans, Ahhiyawa)
Pelestet -> Pelasgoi. It's a much more debated connexion, but pottery associated with Philistines bers likeness with Mycenean/Late Helladic IIC pottery

There is no definitive argument in favour of a large Mycenean component of Sea Peoples, and it's likely that even if we consider this theory, that these groups included peoples from various origin (not unlike how Late Antiquity Barbarians, while "Germanic", included other smaller groups).

Still, it's really tempting to consider that Mycenean groups banded together with Central-Southern European peoples, took an important part thanks to more sophisticated fighting tactics. Why would have they done so, if invaders just destroyed their homes?
Well, it's possible that Mycenean civilization was already collapsing at this point : climatic changes and droughts provoked a significant decline of cereal production in Eastern Mediterranean basin, something that the importantly redistributive palatial economy had more and more trouble to deal with, especially with a crisis of confidence among the population, and a decline of trade with other productive regions (Egypt, particularily) caused by the general crisis. Giving that Late Bronze Age might have seen greaters tensions and fighting between palatial states, it might have exacerbated tensions and resort to force within these states when it came to resolve social tensions.

Even before the fall of Myceneean Greece, it's possible that Greeks mercenaries were in the employ of various palatial states, such as the private guard of Ramesses II was made of Sherden peoples.

This difficult situation might have lead to too big of inner crisis, possibly aggravated by a series of earthquakes, and led to the collapse of Mycenean palatial organisation. Either due to sheer inability to deal with the generalized crisis, either due to inner revolts against the ineffectual wanax (king).
It's not known which role played invaders from the North and West, but it's considered unlikely they caused this collapse by themselves as the narrative of Dorian Invasion could put it, altough they probably dealt the last blow in many occasions, and they likely mingled quickly with local populations and migrating groups on which Myceneans took the lead on several occasions.

One could say that contact with the collapsing Mycenean world is what actually "created" Sea Peoples, out of various groups. Egyptians might have tought so, considering these various "Northern" groups as coming from "the isles".