Not that aliens need a reason to invade, per se, but what was it that set the apocalypse into motion? In The Happening, which supposedly inspired the book upon which Bird Box is based, the attack has a moralistic undertone. The Happening suggests that the attacks are a warning from plants because, essentially, humans are being too destructive. Bird Box doesn't have the same ideology: It's like the creatures are just there for some massive nihilistic prank.