DingerX
I wouldn't go as far to say they lost control of the cabin (it was getting close though as more and more people were up and about as the fumes got worse) but if a passenger did indeed open a door then perhaps they had lost control at that area.
Either way, my own opinion is that it took too long for them to decide to return to the stand. If they had immediately decided to return when the fumes were first detected then I don't think things would have reached the stage they did. Of course, if passengers were disembarking through the jetway then smoke appeared in the cabin it could have got quite nasty.
However you look at it, it was a pretty scary incident and one which I hope to never be involved in again.