Before the asteroid we get snippets of lore here and there that hint at the fact that humanity intruded into Arachnid space and started the conflict by killing lots of bugs and that the bugs started throwing rocks in response to that.

It's a pity the filmmakers leaved out these details from the book. The bugs not only were capable of sending asteroids through the galaxy, but also made starships to travel to other planets. The humans had power armor and carried a couple of shoulder-mounted atomic bombs each.