A tug driver reported the flight crew failed to set brakes after the push resulting in the aircraft rolling uncommanded after the towbar was disconnected until the Captain set brakes.

Narrative

I was instructed to push back a 787 on Gate XX. I connected the plane using a 787 tow bar; inserted the nose gear pin and connected the communication head set; connected the head set to the blue tooth connection device. I have pushed back on this gate very few times and have never pushed back a 787 aircraft on this gate. I pushed this plane back to the proper location center line on the tarmac. When I came to the stopping point; I communicated with the flight deck and asked them to set brake." The pilot indicated that he wanted to be pulled more forward to the front of the tarmac. I then pulled the plane another 20 feet and then asked the pilot to "set brake; clear to start engines." The pilot replied "brake set" I gave the signal to my Wing Walker to disconnect the plane. The Wing Walker then tried to disconnect the tow bar from the Push Tractor. The pin did not come out; so when that happens; the pusher will move the tracker slightly forward and reverse in order to release the tension on the pin. When he was still unable to disconnect the towbar; the Wing Walker released the tow bar from the nose gear of the plane. When this happened; the plane began to slowly roll backward. As I was still connected to the Flight Deck; I immediately said to the flight deck; "Did you set your brake" The Captain replied "OH.." and he set the brake on the plane and it stopped rolling. Then I communicated with the pilot that the pin was removed and the plane was detached. He asked me if he had enough room to taxi. Since we were the first in line at the front of the tarmac; I indicated to him that he was clear to taxi. At that point; the flight deck seemed to be communicating with someone; other than me; and at that point the outside Supervisor showed up at my tractor. He indicated to that I should have pushed the plane to the W line. I asked if he wanted me to move the plane. The answer was no; and that I should back the tractor away from the plane. I backed the tractor to the gate; waited for the outside Supervisor to come back to the gate. The outside Supervisor was going to go back and talk to the flight deck. I do not know what was discussed. The plane then departed and myself and the supervisor went into the office to discuss the details of what had happened. After we reviewed the details; I returned to work."