... Rather than just keep banging on with their own agenda!
If you're not taking the facts of the OP or developing discussion into account, what's the point of going off on a tangent, other than as a deliberate effort to derail? If said poster felt that strongly, surely they would start their own post instead of make it difficult for others to comment without some irrelevant reply.
I'm all for discussion, this being a public forum after all, but there's a big difference.
Or are derailers a specific type of troll?