Scene Context: During deceleration, the crew experiences 2G, 4G, 5G, and a peak of around 6G, suffering from a greyout, breathing difficulties, and near-total loss of arm strength.
Essence of the Phenomenon: After prolonged exposure to weightlessness, the cardiovascular system, muscles, and vestibular system no longer operate in an "Earth-normal" mode. The return of high G-loads is tolerated by the body far worse than during launch.
Scientific Basis: Martian entry trajectories can produce high peak G-loads; the MSL structure was actually designed to handle double-digit G-peaks in certain abort modes. A value of around 6G for a harsh but still controlled entry is not unrealistic, especially for an exhausted crew after a long flight. Concurrently, microgravity causes cardiovascular deconditioning, severely degrading the body's ability to tolerate returning G-forces.