You're correct, and I didn't mean to make it seem as though it was an intentional process.

But historically, when one aspect of the ecosystem goes unchecked, something usually comes along to take advantage of plentiful biomass, one notable exception being humans.

Yes, except even herbivores occasionally show evidence of eating meat. Deers are occasionally seen eating birds, similar has been seen in Cows (IIRC Lal the cow was documented eating its owners chickens from the coop) and in fact all Bovid species occasionally show eating birds.

If there's a world of untapped food, this exceptionally rare behaviour would rapidly become advantageous, especially when the prey isn't adapted to the unusual predator.

Evolution has no guide, but all evidence shows it evolves into almost every available niche and quite specifically that any large available resource will be tapped.