Whilst this does genuinely happen, the term is used by some paranoid people to effectively mean "anyone who does not agree entirely with the standard dogma of position B, thus must actually covertly support position A", when said person does actually support B, just only 99%.
Thus: disagreeing *at all* with the standard dogma of B will get you branded by such people as The Enemy, and you will get generally treated with hostility and suspicion and, at worst, forcibly ejected from the B-supporter group. Result: everyone stops thinking about the issues, and discussion descends into not arriving at logical and interesting places but who can most furiously support B.
2: "Uh.. that's a bit excessive. I mean, I don't agree with the Democrats, but maybe we shouldn't actually, like, KILL them.."
1: "CONCERN TROLL! CLEARLY YOU ARE 100% DEMOCRAT! BEGONE, SINNER!"
(everyone flings rotten fruit at 2 until he leaves)
Supposedly, the idea is that the "concern troll" is actually an adherent of some other, opposing orthodoxy, disingenuously pretending to be sympathetic to the goals of the forum in order to disrupt it or sow dissent. Perhaps this actually happens (this longtime forum user is skeptical, having NEVER seen a clear example), but usually, the accusation comes from someone who can't imagine honest disagreement with his or her favored ideology, and thus says more about the accuser than the accused. Basically it's a bludgeon used by people with very black-and-white views on some topic, against anyone more nuanced than themselves.
Accusing someone of being a concern troll is generally a bad idea. Even if you're right about the "troll"'s motives, which you probably aren't, that doesn't make the "troll"'s arguments wrong; in other words, calling someone a concern troll is a basic ad hominem fallacy. The term is used to shut down, rather than to advance, discussion.
(Noting that facebook isn't a good vehicle for protest makes one a "concern troll.)