Make absolutely sure that your throttle is pulled all the way back to idle when sitting on the carrier deck. The instant that any thrust is produce, the airplane will start moving. If your throttle setting isn't high enough to produce enough thrust to accelerate forward in the 33 mph WOD, the airplane will roll backwards.
Fidd, check your throttle calibration to be sure that pulling the throttle all the way back does indeed set the engine to idle. If throttling all the way back is not setting the engine to idle, you will get the exact problem you describe.
Hope that helps.
MiG