Captain reported the aircraft apparently rolled backwards uncommanded after gate arrival. Captain was informed the ground crew had connected a tug and was pushing the aircraft back without communicating with the flight crew.

Narrative

I was a student Captain upgrade in leg 5 of IOE (third leg with the LCA). I taxied into the gate. Shutdown was normal. The Ground Crew made contact with me in the cockpit; and told me to release brakes. I complied. I turned to face the other Crew Member to continue debrief; and he alertly said 'You're rolling; set brakes'. I immediately re-set the brakes. After confirming the aircraft wasn't moving anymore; the Instructor got on the intercom and made contact with the Ground Crew. He told the Ground Crew we were rolling backwards and the brakes have been set again. Ground Crew came back with a response to the effect of 'The chocks aren't in.' The comment startled both of us. The Instructor responded on the intercom with a response to the effect of 'Why would you tell us to release brakes if there are no chocks in?'; and the Ground Crew responded with 'I am pushing you back. A tug is connected.' This was the first time the cockpit was informed of this fact. The rolling the Cockpit Crew experienced was a tug pushing us back; but we were never informed of this until we were moving. Once we knew what was happening; with the Ground Crew's permission; I released the parking brake a second time; and the small push back continued. Once chocks were in a second time; the Ground Crew asked us to release brakes. I complied.A lack of communication from the Ground Crew about their intentions. Once briefed to the Crew; the situation was handled with professionalism.All it would have taken was the Ground Crew to inform the cockpit of what was going to happen. We were debriefing in the cockpit; and once we were told to release brakes; assumed that was the end of it.