Was the engine at idle? If it was above stall speed then you have a definite issue. Otherwise, an auto trany will not produce enough torqe at idle. Its like pushing in on the clutch pedal, there isn't anything linking the output to the input so it will roll backwards. Angle of the incline will affect how much rpm you need to overcome this.

Every Automatic I've driven does this and so does my X. It does it if I don't give it enough gas to overcome the curb on my driveway and I "bounce" off it.