Why your payment includes taxes and insurance.
Most mortgage payments bundle more than principal and interest. An escrow (or impound) account is how your servicer collects and pays your property taxes and homeowners insurance for you.
Each month a portion of your payment goes into escrow. When taxes and insurance come due, the servicer pays them from that account — so you're not hit with large, irregular bills.
Once a year your servicer reviews the account. If taxes or insurance rose, your monthly escrow (and total payment) can go up; if you overpaid, you may get a refund. This is the most common reason a "fixed-rate" payment changes.