Sim being moved in with another Sim while I'm playing a different family

Can anyone help me (with, I think MC Command Center)? I had a young adult sim split from the household and purchase a house with all the money they saved. I played them a day in that house. I then went back to the original family to play and immediately got a pop-up saying:

“With their romance going so well, Sim Name has moved in with Sim Name! This could be the start of a beautiful future.”

So my sim moved out of the house they had saved for, moved in with another family to be with this other sim, and lost all their money. 

I moved them and the sim that they decided to have a relationship back into the original house, but he lost all his money (I think I remember how much he had before, so I will have to cheat it back)... The reason I moved the other sim in with them, is because just previous to that, I had the same message, but it was another sim moving in with them. This sim is a serial romantic; they do not want to live with anyone or have a serious relationship. And until I play him again, I don't want him moving out and losing his house! 

So what did I do with these mods that allowed this to happen? It has never happened before. Sims in my played households usually stay put, and nothing changes in their lives till I alter them.