Air carrier Captain reported an unannounced tow bar disconnect during pushback resulting in an aircraft breakaway. Captain realized the situation and stopped the aircraft movement enabling the tow crew to reconnect headset complete a safe pushback.

Narrative

Aircraft X Gate XX in ZZZ. The aircraft doors were closed and the jet bridge had been pulled away. We are on a 13 min delay due to a passenger removal and now we were ready to pushback. ATC cleared our push and I informed the ground crew. We initiated the push when in just a few seconds later I hear 'brake' through the headset. It seemed like it was coming from Ramp Control but their frequency became congested. I looked around outside and noticed the gate agents standing at the edge of the jet bridge making gestures to stop. We also saw a ground crew making signals to set the parking brake on. I tried to call the ground personnel but there were no answer. I looked at the Engine and Warning Display (E/WD) and it still showed the NW STRG DISC memo. Then I looked down and saw that the tug was stopped. It was not moving and then as the airplane kept slowly moving backwards I was able to see the front of the tug and noticed that the tow bar was not attached to it. Immediately I realized that the tow bar had been disconnected from the tug for some reason and that the aircraft was moving backwards without any control from the ground crew. We were just a few feet away from our originally parking position but knew I couldn't delay on my reaction because not so far behind the XX gate was a wall barrier and our aircraft was an A321. I also couldn't stop violently because the flight attendants were up doing their presentation so I immediately started stepping easily on the brake pedals until the aircraft got to a complete stopped. I would estimate the aircraft probably rolled back less than 10 ft. and we still had a good clearance from the back wall. I kept both of my feet on the brakes and waited for the ground crew to reconnect their headset. As soon as he reconnected the headset he confirmed that the bar had been disconnected from the tug. I asked if anyone got hurt and if there were any sort of damage to the NW STRG connection; the nose wheel itself or in anywhere else. He confirmed no one got injured and there were no damage anywhere whatsoever. I asked if I could set the parking brake and he confirmed yes. Parking brake was set; tow bar was reconnected to the tug; we informed Ramp Control and continued with our pushback with no further events.