im not gonna assume you know anything so i'll ask, were you pushing in on the brake/clutch lever?
remove the deck and get a helper to work thew brake as you look under the mower and watch how things move when you press the brake lever, im sure the issue will be obvious as how it all functions to release tension on the drive belt
i gotta ask because i had a guy with batteries going dead for no reason and turns out after he killed it by turning the key off, he always turned the key back on before going inside the house. and this guy was in his late 30s and apparently was clueless how mowers work right to adulthood
