The bow down attitude is the result of restrictions in the angle of the rudder foils and the different speeds upwind vs. downwind.

Upwind, the boats are relatively level; the boats are moving 25-30 kts. A higher angle of attack is required on the rudder foils to lift and stabilize the back of the boat.

Downwind, the speed increases to 35-40 kts; a lower angle of attack on the rudder foils is required to achieve the same lift, so the boats assume a bow-down attitude.