Twice a year Mastercard and Visa complete a “semi-annual regulatory” update (aka rate assessment) on all cards. The result of this is either an increase or decrease of Interchange pricing associated with the cards in circulation. The cost, or savings, is passed on to the merchant. This is perfectly normal.
Why is this important?
Many processors will use the aforementioned as an opportunity, or an excuse, to increase your original agreed upon rate. Which in effect puts the blame on the credit card associations. What they’re actually doing, though, is using this as an opportunity, and an excuse, to line their their pockets.