I generally find lifeforms that count on magic creatures like golems or the like common enough that things evolve to prey on them rather unlikely. A better explanation is this a weapon built to destroy such creatures, which opens up the Who and the why questions.
So accepting this is a magic weapon to destroy golems, having a side effect of conversion into monsters is somewhat more plausible, but simply leaving a patch of green where once stood a golem, or a half-consumed golem lashing out blindly are less odd to me :)
Not every prophecy needs to be meaningful to effect a game. In the Lord Dunsany play, The Golden Doom, a child's scrawl has an entire kingdom struggling to puzzle out what sinister prophecy it portends.