Yeah, the way I understand it, the AI will send Zeds walking around to re-populate areas where they've been thinned out. Chances are, if you've thinned out the Zeds around your base, you're making it a more attractive position for the AI to send zombies to. Thus combined with this noise generation, and meta event noises, you might eventually have a swarm showing up on your doorstep. It's not the player built structures per se, but a combination of different factors that will cause this, and they are somewhat plausible from a simulation standpoint.