I read in a thread somewhere that the Necrons teamed up with tau to destroy some tyranids. The Necrons may feel that sense the tyranids come from beyond the galaxy, that they are one of the greatest perversions in the galaxy.
My own personal opinion is that the Old Ones retreated to beyond the galaxy and created the tyranids, in order to set them upon the galaxy to completly wipe out all life, so that the Old ones can start again. Just my personal oppinion, complete conjecture.
Fuck the Meta-game