You had MCCC installed. MCCC moves and adjusts households. You installed a mod without undersatnding what it did, and it did the things it is designed to do. It has its own story progression features that move Sims around following a set of default rules that you clearly did not adjust, since you don't understand how any of this happened. Lesson learned: Don't install mods without understanding what they do.
This has nothing to do with neighborhood stories or with talking to an NPC. You simply installed a mod without paying attention to it. The population module filled empty homes and moved Sims around. The Pregnancy module added a ton of babies. It's what these mods do.
Intern_Waffle_64 that was my first thought but they said they installed MCCC after it all happened. Which is why I'm confused.
I'm betting they simply didn't notice until after uninstalling the mod. The *instant* MCCC is installed and you run your save game in live mode, it starts doing tons of stuff.
