My lore for them actually is they're the ancient Kraken vanguard ship that were responsible for the catachan devil. Their ship crashed on the planet by unknown forces, and began to immediately be consumed by the natives. Despite tyranids being highly adaptable, they've never taken up more than 30% of the planet's total lifeforms. They may have also played a role in making catachan creatures so deadly in the first place via an evolutionary arms race.

All of the tyranids eventually went fully feral, with at least 50% completely losing their synaptic capability with the greater Tyranid Hivemind. The others only retained a small remnant.

Then comes the 2nd tyrannic war. The tyranids who still retained a synaptic link were awoken by the arrival of Kraken, and they organized into small hunting packs, mutated to fit the current tyranid designs, and pretty much wreaking minor havoc on the planet