Board Thread:Warhammer 40k General Discussion/@comment-86.7.138.66-20130615215334/@comment-4851119-20130922154218

Theory:

The Tyranids were a backup system left somewhere across the universe by the Old Ones, basically a slow-moving onslaught that would eventually arrive to destroy the galaxy, but would only start moving upon their extinction. The Nids were so much further away at the time this happened that they took millions of years to arrive instead of thousands as was planned, why isn't known. Along the way some moved faster than others causing them to split up into distinct Hive Fleets. The first of the Genestealers arrived far ahead of everyone else (again for reasons unknown), followed many millenia later by Behemoth, Gorgon etc which blindly raced far ahead of the main assault. Kraken was the recon movement, the ones that came after were stragglers who were too slow to keep up the main scout party. Leviathan and Medusa are the first waves of the main invasion, intended to strip every remaining species of the galaxy of all miltary resources so that they have nothing left to oppose the final Tyranid assault with.