Same thing with Enemy Race Manager I'm using, was chilling on Lignumis about to leave the planet and got hit by a massive wave... reloaded and I'm able to sustain now (tons of wooden walls and gun turrets, however that'll only last so long since I cannot set up auto repair until I'm coming back from Nauvis with the tech researched...). ERM triggers waves when evolution is over 30%, maybe Rampant does something similar. I don't know if either Lignumis or ERM settings can prevent biters appearing on Lignumis... :-/
My plan now is that I have placed a ton of new and healthy walls and gun turrets and other stuff in some sort of parking lot, and when defense or other stuff gets destroyed (which will happen inevitably), I'll move some stuff from the parking lot to the border via Even Pickier Dollies while I'm on Nauvis... bit cheaty and I'll probably give up the hassle and rebuild Lignumis base from scratch when I return, but that's the best idea I had...