Air carrier flight crew reported while troubleshooting an EICAS message during engine start; the aircraft started to roll and the flight crew applied the brakes to regain control of the aircraft.

Narrative

Hardstand engine start; pilot monitoring (PM) and pilot flying (PF) were dealing with a startling EICAS event (takeoff config warning) during engine start. Also; there seemed to be a slight hung start on #2 engine (possible cause of the EICAS). I was in back corner jumpseat so did not see this but heard the flying crew discussing the matter. Out of the corner of my eye I saw the aircraft start to roll and I called out we're rolling; we're rolling!" and brakes were being applied as I got the second warning out. Pilot flying confirmed with ground that everyone on the ground was ok; and the rest of the after-start checks and taxi out went uneventfully.Root cause: crew startle/distraction event coupled with a communication breakdown between ground crew and flight crew. Captain felt he heard "release brakes" from ground crew and did as he was advised and even confirmed brake release as per pushback SOP; although it was not clear to the flying crew if we were pushing back or merely taxiing from the hardstand"