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I like the idea that the Tyranids were a bio-attack from one galaxy to another - not intended to survive forever, just to eat one galaxy and starve.

I think that the the Old Ones (which we know were capable of all sorts of magickry) may have started a war with another galaxy, and then that galaxy sent the tryanids as a bio-attack on the Old Ones. The Old Ones then built the Orks as their counter-attack, but were scuppered by the War in Heaven with the Necrons (which, if this bit of fluff survived the retcon, were the Necrontyr, hated the old ones and the old ones basically ignored them until it was too late).

The Orks (a bio-engineered race which thrives on warfare and endlessly regenerates unless an exterminatus is performed on the planet) had been spread throughout the galaxy as a defence network to engage the tyranids and halt their progress - hence why Orks are found everywhere, even in unexplored parts of the galaxy.

So yea, I think the Nids are a bio-engineered superweapon for wiping out potentially hostile galaxies. Maybe they are autonomous, released by an ancient "peacekeeping" race in a distant part of the universe to wipe out potential dangers before they occur. This would explain why several fleets arrived from different directions to a galaxy where there is only war! it would have drawn them all in like a moth to a flame!