When a vehicle stalls in any gear, it's generally a problem with the amount of fuel, amount of air or electric spark getting to various parts of the vehicle's engine. An occasional stall while in reverse doesn't necessarily mean there is a problem with the vehicle, though repeated and frequent occurrences may warrant investigation by a mechanic.
Stalls involving a manual engine are often driver related. Failure to press the clutch or put the vehicle in proper gear can result in an engine stall in forward or reverse. Depressing the clutch too quickly may also result in a stall. An automatic engine may stall if the vehicle is moving the wrong way for the gear, so starting a reverse gear when a car is facing down on a steep hill may result in a stall. To recover from a reverse stall, leave the car in gear. Put the handbrake on to keep the car stable. Restart the car and remove the handbrake before moving.