User blog comment:MemoryAngel/Confused Ablut The Serpentine (Again!)/@comment-27747281-20160210004541

I think it was a combination of them trying to force the humans away and Chen manipulating both sides. Also, I don't think it was necessarily stated when the Great Devourer was used, although I'll go out on a guess and say it was a last resort, as Acidicus stated that after it had failed, that was when they got entombed.