They were part of a minor Convoy that was "leaked" to rogue Grand Admiral Zaarin, claiming to be carrying Missile Boats. In reality, they were carrying munitions. Zaarin, however, learn't that it was a trap and sent only a minor force to destroy the convoy rather then capture it. The attack failed and the two container transports jumped into Hyperspace.
Behind the scenesEdit
The transports appear in Star Wars: TIE Fighter in Battle 11, Mission 3. Oddly enough, the breifing states they are carrying munitions. In the game their cargo says "Miss. Boat" making it seem that they do carry Missile Boats.