Tug Operator reported during rain shower while connected to aircraft with tug brake on; both started an uncommanded movement resulted in loss of control.

Narrative

I was waiting for the flight at gate to get brakes released on the tug. Once we were given brake release; I gave the signal to the wing walker and he proceeded to capture the aircraft. The flight deck called back to tell me we were second in line for push back. The rain started to come down harder and started to soak me through my clothes. I had the proper Personal Protective Equipment (PPE) on; but I was starting to get drenched. I then went to get cover under the jetbridge. As soon as the plane continued to be captured; the plane and the tug started to roll slightly. As soon as I noticed the roll I hopped back on and double check the brake button; it was fully engaged and still was rolling. It then stopped. this casually happens on the vehicle if the ramp is wet. Never have I ever been called to the office for this scenario; so I am shocked. However; right after the push finished; I called it into the controller to call the garage; but I got no response. A user check was done on the vehicle. A incident report (this one) will also be done.