Politics in HK, and indeed everywhere is a strange game. There will be no end to speculation and conspiracy theories on this reform vote.
As experienced a group as they were I think the moment made them irrationale and they lost their nerve. It happens in big moments in sports too, where the player or team which is seemingly cruising to victory throes the advantage away inexpicably.
HK people are not politically savvy and this blunder showed up their stupidity all too well.