Preliminary data from the Indonesian authorities:
The Boeing 737-900ER was holding over waypoint TIRUS which was about 40 Nm north west of Juanda International Airport and maintained altitude of 10,000 feet while the Airbus A320-200 aircraft was about to make holding over waypoint TIRUS. The air traffic controller instructed the Airbus A320-200 pilot to descend and maintain at altitude of 11,000 feet. The Airbus A320-200 aircraft descended and passed altitude of 11,000 feet. The TCAS RA of both aircraft activated and the pilots followed the RA until clear of conflict.