I actually had a sim randomly break up with my sim the other day and that was the first time it had happened to me. I have two playables in different households that started dating. They both were already 'boyfriend and girlfriend' with other non playable sims. I was playing male sim and a pop up notification came up informed me that his non playable girlfriend had broken up with him and she was really angry and hurt. Is it possible that she heard a rumour that he was dating someone else? It was fine, as I had planned to have him break up with her anyways, but I had never had that happen before.