There's a certain breed of grammaitcos, who love to slam their dicks between the musty, gilded pages of gigantic leatherbound dictionaries, who can't help getting their hackles up when they live through the usage/definition of a word changing. Technically, "reactionary" is a stupidly redundant word that only describes someone who is Right-wing, or yearning for the good ol' days of what they perceive as the status quo, or opposed to liberal ideas/politics/philosophies/etc.; simply describing someone as "conservative" makes much more sense and is FAR more relevant these days. The traditional definition would be like if the word "partisan" only applied to one side of the political spectrum; it's a dumb, archaic definition that is, again, completely redundant now.
Most people TODAY, however, use it in the sense of somebody being rash or angry or shortsighted in how they respond to something. I'm assuming the most recent thing here that sent him off was in the latest game thread when people were talking about Theo being reactionary, like whether or not he'd be flipping out/furious over the Cubs being swept by the Pirates at home.
Basically it's a super dumb rant that was just an excuse for him to try and sound like he's smarter than everyone else.