if you actually understand credit scores and how they work, it's pretty easy to figure out.
paying off your mortgage is just like canceling a credit card you had long term. doing this removes a LONG term active positive event from your credit history. doing this changes your length of credit reported used to determine your credit score.

a similar credit event would happen if you applied and received a NEW CREDIT CARD ACCOUNT, not a new card from a EXISTING account.

going from a 850 to a 810 won't really make much difference on you financially and that # won't stay that low for to long if you continue to use credit responsibly. if you don't used credit actively this number will continue to get lower.