I've experienced the exact same thing and had the exact same thoughts. When we are all of a sudden being told we are over our limit with 8 days left was shocking, as it was totally not in line or anywhere consistent with our past usage. After paying $75 in overage fees, the very next month we went over our 12GB limit with 14 days remaining!!?? And to top it all off, our Internet Provider (Frontier) lost it's internet connection with us, and we have to wait 3 weeks before a technician can come out to get our internet back.
This certainly sheds some light on a possibility why you had data overages. Even if a router does not have an internet connection, it can still send a wifi signal from the router to your phone. If the phone determines there is not an internet connection even though it has a strong wifi signal, it will choose to use cellular data even though it IS connected to your router via wifi. Perhaps you had lost your internet connection with Frontier before you realized it and THAT is why you exceeded your data allowance with 14 days remaining in your cycle.
Furthermore, if your router suddenly lost ALL internet connection, it seems at least possible you had intermittent internet connection for at least a period of time before ALL connection stopped.