That's exactly what drag is! Drag is a force that acts rearward to an airplanes direction of flight. Ignoring the slight angle between the thrust/reverse line and flight path line, a reversing propeller is purely a drag thing.
Well, no actually, it's thrust with a value in the negative range, if you want to play inane semantics. I fly the Herc. We will on occasion, taxi backwards. Do you really think that it is "drag" which is making the airplane move backward?