Board Thread:Warhammer 40k General Discussion/@comment-7186329-20130329201743/@comment-7855563-20130410011214

Chaos doesn't interact with reality unless there is a danger to them on this side; hence the Horus Heresy to destroy the Emperor and the Imperium's greatest weapons. The Tyranid infestation is a danger to the Chaos gods as if they cannot take it, or destroy it, then it will kill everything and leave them without power, and thus life.