It's always been present. I can't construct proof or a solid timeline. However I recall back when comment scores were public and upvotes had much more momentum

It's always been present. I can't construct proof or a solid timeline. However I recall back when comment scores were public and upvotes had much more momentum (the more upvotes the comment had, the more it would get), there was a much-lamented problem of two-bit posts of the sniping variety, ostensibly as a low investment spray-and-pray strategy to try to catch a runaway upvote train. I want to say it actually improved after comment scores became private, though it could have easily regressed since then.