The first stone was thrown when the various TIG MPs left - that was the point at which the policy changed, and the change in policy was a direct response to that - and while in the end TIG turned out to be a stillborn minor split, weve no idea how many would have gone had the policy not changed, or the kind of pressure the leadership was under.
The leadership - and Corbyn in particular - were very aware of the problems this could cause, so we can only assume that the assessment they made was that the threat from not doing it was more severe. Who knows, maybe it was? As I say, they were fucked one way or another whichever way they jumped.