I heard the unconfirmed news saying that the Korean bought the drone parts from Shenzhen, China and assembled the drones by themselves, then wrote their own control codes. The incident was actually caused by the buggy codes that the Korean wrote and the result was a big disaster. On the Vietnamese side, did not know why they did not hire the Chinese drone team for the demo. The original plan was to use 3000 drones but the Vietnamese wanted the Korean to add more drones to the show, and the 1050 more drones were added. Maybe some codes were added in a hurry to control the extra drones, which led to a bigger mass.