seems a bit linkbait-y.

i would have guessed, 'competition due to more-limited resources, due to direct or indirect human encroachment and environmental destruction'.

and that seems to be what the article says, but seems to leave out any mention of humans, so maybe the animals' resources are just disappearing due to ???

Paywalled: https://archive.md/sByqF

Why kill infant gorillas, so they don't grow up to outnumber the chimpanzees. This is warfare to attack the youth and let the old die out.