A Manchurian Troll is basically anybody who is normally perfectly fine, humane, and hospitable up until they hear a specific triggering keyword which brings out their troll programming. Usually the transition takes place as a rapid and violent conflagration.
The name itself is a reference to the book and film titled The Manchurian Candidate, about a normal man who is brainwashed into being an assassin.
Hey man, don't talk about gay people around Charles. The dudes a major Manchurian Troll and that's one of his triggers.