B767 Captain reported temporary loss of hydraulic pressure and subsequent uncontrolled movement of the aircraft on the ramp. The First Officer mistakenly had shut down the hydraulic pumps; causing loss of hydraulic pressure.

Narrative

Parked in assigned slot at ZZZ. FO (First Officer) started APU on taxi in. I noticed this when we parked so I shut down both engines. I elected to hold brakes because we had limited time on ground and were scheduled to take the same aircraft to ZZZ1. We had brakes temps on the right and we waited for the Ramp Personnel to chock the aircraft. The Ramp Personnel were taking a long time to chock the plane. Suddenly I felt the brakes become 'squishy' and we started to roll backwards. I looked at the gauge and realized we had NO pressure. The Mechanic and Ramp Personnel were screaming at us as the airplane rolled backward. I was dumbfounded as to what had happened. Feeling helpless I tried to figure out what would cause this and looked up. All of the hydraulics were shut off! I asked why this was like this and the FO stated that he had performed the shutdown and secure checklist. I had NEVER called for this; as we were not even chocked in. We turned the hydraulics on and I was able to stop the airplane. We had moved back several feet and I had to restart the right engine to pull forward. I am writing this so that it does not happen to someone else. Through no action or direction of my own; I would have been responsible for possible personnel or equipment damage. This highlights the need to only accomplish these items when they are called for. I routinely operate into ZZZ2 followed by a turn to ZZZ3 where we ALWAYS hold the brakes and NEVER set the parking brake because we will damage the brakes. I was completely blindsided by this because I did not see the action completed. I was watching outside; waiting for chocks in signal. I am just glad we saw this and were able to avoid a mishap. Nothing was damaged; no violation occurred but hopefully this will allow someone else to know where to look should this happen and we can prevent it from occurring with reminders that these actions should not occur without direction. Thank you.