Under normal circumstances the Gorm are arguably something of an inverse of the Devourers. Devourers are expressly said to be intruding upon the realms in search of anima to consume, so while their recent proliferation isn’t the norm, their actual behavior is.
Ardenweald quests indicate that Gorm ordinarily feed on matter that doesn’t have anima left in it, effectively serving to break down and recycle the depleted material that remains after reborn souls move on.
But more recently in the absence of fresh anima, Ardenweald’s very landscape has begun dying off, leading to a glut of lifeless material and consequently an explosion in Gorm population. Such that even with the excess depleted matter, the Gorm numbers have grown even greater still, leading to some of them consuming material that still contains trace anima and even an occasional soul as they scour the land for sustenance.
And the implication is that when Gorm eat anima (or worse still, a regenerating soul), since it’s something they normally wouldn’t do it affects them unnaturally, causing them to greatly increase in size, strength, aggression and intelligence while causing them to hunger for more anima.