Suspect its because a number of douchbag Labour MPs, Brexit not withstanding, have been living under the threat of deselection anyway, even before this latest charade. So now the gloves are off and they can vote as their constituents did on Brexit, presumably believing they will at least have a chance of getting back in at the next GE as 'Labour for Leave' or even perhaps Brexit Party MP's.
In essence they will now see themselves as being 'fire proof; hence' no whips threats can affect them now, they are already beyond that effectively as Labour PMs they are now on 'death row', at least as far as certain sections of the Labour party is concerned.
Jeremy has been repeatedly warning his party not to shoot themselves in the foot and now he's so close to a GE, he must be tearing his hair out!