Most people have an escrow account with their mortgage. That escrow account adds to the monthly mortgage payment but then provides funds to pay property taxes & homeowners insurance. The cost of taxes and insurance might vary from year to year, so an annual analysis of the escrow account is done, snd may result in an overage or a shortage. This is typically regulated by state law. When people experience a change in their mortgage payment, it’s usually* because they had an escrow shortfall

*I say usually because it’s also possible to have an ARM mortgage and the interest rate changes based on the terms when the mortgage started