Air carrier Captain reported the aircraft rolled back after a ground crew trainee called for brake release before the tug and tow bar were in place. Captain applied brakes and stopped the roll.

Narrative

Before push and door closure my FO (First Officer) left the flight deck to use LAV. MCD and FWD CARGO were still open and I heard ramp call me over the intercom. I ignored the first call. He called again asking if I could hear him. I said loud and clear and he said they were all complete down below and ready for brake release. (Something along those lines). Seeing as we had doors open still and no Before Push checklist done I told him we needed some time still. At this time I thought it was odd that he made those commands with MCD and FWD Cargo door still open. I didn't feel the need to address it because I wasn't ready at that time anyway.A few minutes later the FO returned to flight deck. I had final paperwork; and gave authority to close MCD. Moments later FA gave me the ready to taxi thumbs up and we closed FD door. At that time I noticed FWD Cargo still open. We sat for a very short period and I saw the FO reach for the checklist I had not called for. I looked up and saw all door lights extinguished and called for Before Push Checklist. Halfway through the checklist; around the Hydraulic Pumps call I heard ground over the interphone saying 'We are all set below; we are ready for brake release'. I ignored the call for a few seconds as to not interrupt the checklist. Seconds later; once before push checklist was complete; I responded; and simultaneously released the parking brake and said; 'Brakes released; standby for pushback clearance'. I recall the FO being heads down sending an ACARS prompt. My FO also said; I hate that feeling when you are heads down and you feel the aircraft moving and its just the jet bridge; I laughed and said I remember that feeling or its another plane pushing back next to you and its the parallax of your peripheral vision giving the illusion of you moving. I fumbled around for the next few seconds either putting my headset on or looking at the FMS when I looked up and saw a frantic ramper jumping up and down giving me the set brakes hand signal. I immediately applied brake pressure with out slamming them as I realized my FAs were probably standing. I then set the parking brake.The next 60 seconds was the darkest; most fearful moment. I saw a ramper in anguish throw his hat against the ramp floor; another ramper on his radio acting frantically. I saw a pickup truck driver with his phone out the window recording. I told my FO; either someone is dead or seriously injured given the frantic responses and movement from the ramp personel.In what seemed like a minute I saw a crowd of ramp crew out my window looking at my airplane and I still had no verbal communication. I opened my window and yelled over and over but nobody could hear me. I hit the ground call and got the attention of ramp crew and asked if anyone was hurt. He said no. This was my biggest relief. After that I asked my FO if he also heard the release brakes call. He told me he heard them call but didn't make out what was said because he was focused on the checklist.In that moment; I was 99% sure I heard the release brake call (it was the second time he told me in a few minute period). However given the adrenaline I started to question maybe I had confirmation bias as to what I heard. I then called Chief Pilot and explained the situation. He said to fill out reports and to stand by for a call back on whether we could depart. We got a call back shortly after saying we were good to go after I gave a verbal report as to what happened. At that time a new push crew called ready for brakes release and push but I asked for clarification as to what happened. He said a trainer was talking to me. I asked to speak to him but he was gone and he left and let me speak to a trainer.The trainer told me that when the brakes were released the plane was not in the chalks and we were not connected to the tug. I then asked why did he give me the release brakes command? He said he wasn't sure; he was being trained but he shouldn't have given you the release brakes command. He verified that he gave the command to release the brakes. I then asked if he was hooked up to headset why did he never give me the 'Breakaway Breakaway'; command. He responded he did not know but suggested he may have panicked. Despite serious frustration I told them I was grateful nobody was hurt and asked to be put back on with the new push driver.We pushed without incident after getting a new push clearance.We are lucky I looked out the window when I did; we were moving back at a good pace and the service road is not far behind gate with cars and employee buses. More seconds wasted would have transgressed the service road and further the [Taxiway] 1 line on ZZZ ramp. Secondly; had the ramp not been on a decline it would have been seconds that we crashed into the cart/tug; or the terminal or other ground equipment.