I know, I know, why install rampant AI if you're going to play in Peaceful mode? The thing is, I have so many mods installed right now I wasn't thinking about it much... and maybe it's another mod causing the issue.
At any rate, I decided I wanted to play without alien attacks, and tried to turn on peaceful mode using the console command (followed by the kill all aliens, I know how it works). The attacks kept coming.
Then I started a new map, specifically selecting the "enemies never attack first" option. I had a Very Large starting area, so it was a long time before I noticed -- but suddenly, waves of aliens started attacking a mining base.
The only mod I can think of that might be causing this is the Rampant AI. I'm going to disable it and see if the issue persists.