Well, aliens can assault your base multiple times in LW. There's no special action that triggers that (like you assault their base, which happens in vanilla). They can assault whenever they want, or when they think you are a threat to them, which means, at any moment. I had base defense before I did the alien base assault. So, if you see a large UFO heading to your base, prepare your ass. Anyway, I read some comments around to avoid Ironman when playing LW, cause it may contain bugs that break the game.