The operator reported that while conducting a high-altitude airdrop test, the 25,000lb airdrop package prematurely released while the airplane was maneuvering. The airdrop package extraction parachute did not deploy, and the package exited the rear of the airplane slowly, resulting in an excessive pitch-up and right yaw of the airplane. After the airdrop package exited the cargo door, the airplane pitched down violently, resulting in serious injuries to two crew members located in the cargo compartment. The aircraft returned to the airport without further incident. A post accident inspection revealed no aircraft damage and that the cargo locks were set improperly during loading. The operator reported that the airplane had no pre accident mechanical malfunctions or failures that would have precluded normal operation.