Air carrier Captain reported a non-SOP pushback without headsets resulted in a tug not connected to the aircraft with brakes released and uncommanded aircraft movement. The Captain noticed the aircraft rolling and quickly set the brakes as the aircraft rolled into the tug cradle. A maintenance inspection was performed and no damage occurred.

Narrative

While passenger boarding was taking place; the lavatory was being serviced and someone inadvertently separated the external dump valve latch; which required Maintenance to reattach it. We called Maintenance and then the ramp closed due to lightning so maintenance couldn't come for maybe 20 minutes. Once they arrived; they quickly resolved the issue and closed the write up. We closed the main cabin door at XA:31 and waited for the push crew. We were advised by ramp control that headsets would not be used by any push crews due to the weather; so I did my best to overcome that potential threat by using standard hand signals and also typing instructions into the 'make it big' app on my tablet. While waiting for push crew the ramp closed again due to lightning. It was closed for nearly 45 minutes. Around XB:10 the ramp reopened and a manager sat down in the tug and we established that the chocks were removed and we called for push clearance. They instructed us to give way to one outbound and then we were clear to push. I then gave him the hand signal for brakes released and using my tablet I wrote to give way to one outbound then cleared to push. Tug driver used his phone to show me he was waiting for a 4th person to push so I advised him using my tablet that once that 4th person arrives we were clear to push. Literally within a couple of minutes the ramp abruptly closed again and the driver got visibly upset and threw up one hand and jumped off the tug. It was pouring rain and I didn't notice that he disconnected the tug until he started moving backwards. He didn't install the chocks; so the plane started rolling slowly forward and when I noticed I quickly pressed the brakes and set the brake handle; but not before the nose wheel rolled into the tug cradle. We wrote the incident in the logbook and when the ramp reopened Maintenance inspected it and signed it off. They said it landed exactly where it's supposed to be and there was no damage. Just as we were about to push back (approximately 90 minutes into the delay); a gate agent unexpectedly insisted on attaching the jet bridge to come on board and make an announcement asking if anyone wanted to deplane. I expected the ramp to close again at any moment and am glad that additional gate agent request didn't hold us up long enough to be affected by another ramp closure. We closed the door approximately XC:16 and departed without further delays or incidents.