Q: Why does my Nissan Xterra roll backwards on a hill when I release brakes?

A hill that is more than a 10 or 15 degree slope may be more than what most automatic transmission can handle. It would help to know what type of engine and the year of your car but an automatic transmission is no different than trying to start off on a hill with a manual transmission.

Your transmission still has to engage the first gear clutch and with the proper engine speed held it will keep you from moving backward at the expense of the first gear clutch getting worn. If you do this on a regular occasion then you will wear the first gear clutches to the point that at the lower pressure of idle they will not hold the vehicle on a hill. The vehicle will move backward until you rev the engine up and increase the transmission pump pressure on the first gear pressure plates, then the vehicle will start to move forward.

The problem you have is worn clutch discs or low pump pressure and would need to have the stall speed tested on the transmission. In the mean time, hold your foot on the brake and using the other foot raise the idle when you want to start to move up the hill. I would suggest having a professional technician, such as one available from YourMechanic, diagnose the cause of your issue.