what does the term sandbagger mean?

In auto racing, it’s a similar deal. If you’re not pushing your car as hard as you possibly can, you’re ‘sandbagging’ – or witholding some additional performance.

As if the car were loaded with sandbags that could be easily removed to increase the performance.

I feel this is the reference you’re looking for. “Sandbagging” refers to witholding some effort or not trying as hard as one can, as if bags of sand were holding one back.

In bowling a sandbagger is someone who has a high average (i.e. 200 scoring average) and then bowls a 175 average on purpose. In handicap leagues a bowler will do this for the first few week until his/her average is established (getting extra points added to their final score). Then the rest of the season when they need to bowl extra good they can do so and still get extra handicap points. Sandbags hold things down or hold them back hence the name associated with held back performances.

I hadn’t imagined this term being used in the Marines, as I’m familiar with the term from playing cards. When someone takes more tricks than they bid it’s called sandbagging. It seems to me that the same meaning applies to your example from the Marines as does in card playing: “To underperform in a race or competition in order to gain an unfair handicap or other advantage.”