Crate Capture is an unreleased gamemode that was available to play on Buccaneer Bay around June 2010 on the PTE. From this time on it got lost in a dimple and hasn't been mentioned again beside some quotes:
During the round, a crate drop would be spawned in the air, parachuting down to one out of many locations. On each players screen, a message would describe where the crate would land. Players then rushed to said point, cleared it out of enemies and then secured the crate. If no team captured the crate within 3 minutes, the crate would be considered lost. 7 Crates were dropped in total. Once a single team had captured 4 crates, they were declared victorious.
However, a notable flaw is that tanks could cap the crate.