Hi Jenn, when you are returning to the starting position are you relaxing your body? You've said that you are returning the weight slowly so I gather your upper body is still somewhat tensed, but are you relaxing your legs?
I find that when I'm pulling down my lower body also tenses and my legs push up against the retention guard over my thighs - this motion of my legs pushing upwards has the effect of my Cheeky cheeks pushing downwards into the seat. If you are relaxing this action on the return, this could explain why you are prepping for lift off.
Also depending on your height, it might be that you are trying to let the handle return too far - as you have to stand to start this excercise, you cannot let the handle return all the way to the starting position without some form of standing occurring (unless your arms are incredible strechy). Try not to let your arms go completed straight when returning the handle, try and leave at least some bend at the elbow.
Hope this is of some help! Incidentally how much do you weigh?
Mark