B777 Captain reported the tow bar broke and disconnected during pushback resulting in momentary uncommanded movement before the brakes were applied. Captain stated it was not known if improper tow procedure or faulty equipment was the cause of the event.

Narrative

Prior to pushback; the tug driver asked us to be patient because he was new/didn't have experience pushing back. The push began at XA:21 from Gate XX. Approximately 60 feet into the push; while the airplane was still moving straight back; I heard a bang/thump. The tug driver exclaimed 'set brakes! set…' and then the comm cutoff. As the airplane continued to roll; the top of the tug came into view and it was stationary. I realized the airplane had become disconnected from the tug. I gently applied brakes so the airplane would not sit back on its tail. Once we stopped rolling; I set the parking brake. I immediately called Ramp Tower and told them what happened and asked if the ground crew was OK. Ramp Tower reported things looked normal around the airplane. A couple of minutes later; a different push person checked in on interphone (I believe this time it was a mechanic) and told us the tow bar had broken and all ground personnel were OK. The broken tow bar was removed and the push crew obtained a 767 tow bar and said they could push us with it. I confirmed the 767 tow bar was sufficient to push us AND that the previous tow bar did not strike the airplane when it broke. The push crew confirmed the broken tar bar did not strike the airplane; so we called Ramp Tower and resumed the push. The flight to ZZZ1 was uneventful. The ground delay was XX minutes. I do not know if improper tow procedure or equipment failure caused the tow bar to break.