Air carrier pilot reported uncommanded movement of aircraft at gate. Tug reconnected and flight proceeded with no further issues.

Narrative

The day started as a routine flight. It was still dark and raining which could be a contributing factor. We had just finished boarding and the main cabin door had been closed. While I was NOT monitoring the ramp conversation I was obviously privy to one half of the conversation and the events that follow. The Ramp informed the Captain that they were ready for brake release; however the status synoptic page showed I believe one of the cargo doors still open. The Captain then clarified with the Ramp who closed all doors before the brake was released. We ran the 'below the line' checklist; the Captain then released the brake; and told the ramp crew we were cleared to push. I was then calling Clearance to coordinate a flow time for departure; and we started moving backwards at seemingly a normal pushback speed. Suddenly I heard bleed over from the ramp intercom of the ground crew yelling to set the brake. The Captain immediately stopped the aircraft and set the brake. At that point we both realized the aircraft had moved but the tug was farther away. The crew then informed the Captain that the aircraft was not captured properly on the tug and began to roll backwards apparently free from the tug. The aircraft rolled backwards approximately 5 feet in my perception before we stopped. The tug reconnected and pushback was completed without further incident. The Captain verbally confirmed no individuals were hurt and nothing was damaged. We continued on to our destination with no further issues. The tug in question was one of the strapless tugs that clamps around the nose-wheel tires.