So... I have a guess as to why this is. It might be entirely wrong, but it kind of makes sense to me. I would bet the bank also partially insures your home when they hold your mortgage, and so the two insurers/policies split the liability in that case. When your mortgage is paid up, the bank no longer has stake in the home and doesn't insure it anymore, meaning the full liability falls on your insurance policy alone now. So they likely raise your premiums to account for the hightened risk on themselves. That's not necessarily a justification and it obviously sucks for you, but, it does make some amount of sense IF my guess is correct.
Either way, paying off your mortgage is a big accomplishment and removes a big burden from your shoulders. So kudos on that at least.