The phenomenon is not dissimilar to this: Word for "butting in on the Net", yet it wouldn't necessarily be considered trolling.
Person A replies to a comment/post by Person B on the internet. The conversation might be clearly between Person A and Person B, and it may be a debate/disagreement/point of contention.
As the conversation continues, Person C and Person D (who may be unknown to one or both parties)have been observing the exchange and decide to join the conversation, taking the side of the person they agree with most. In this case, they side with Person B.
So now Person A is having an exchange with Person B, C & D simultaneously.
It's basically 3 against 1. The 'spirit' of the original conversation is now dead, and it has now become a firefight for Person A and more of a witch-hunt for Person's B, C & D.
I see this happening in social media every day. It's a nasty pattern.