Overpayments resulting in unwanted recalculation

CopperKid

I am on the homeward straight and on a low interest fixed rate. I have been making annual overpayments with the intent of reducing the term. In previous years made the permitted 10% overpayment and a quick phonecall to request that the overpayment was applied to end of term. I wanted the monthly payments to remain the same. 
This year a recalculation was applied at the annual review which reduced my payments. I phoned up and have been made to go through a mortgage review which has lasted several hours at this stage. I didn't do anything different this year than previous but I am being made to jump through hoops. I have not missed a single payment, I abide by the rules but something has changed.
Has there been a change of rules that would cause this to happen?