The "Aleppo Moment" was a designed hatchet job by the corporate press. It happened right after the first polling showed him taking more support from Hillary than from Trump.

The interview had been very friendly and was not even remotely touching the topic of ISIS, yet. Then this deep state hack abruptly asks "What would you do about Aleppo?" Normally, even if only for the audience's sake, there's a transition between topics that gives people a moment to get their thoughts in order. They never just dive into a hyper particular element of a topic like that.

Also, all of the previously smiling hosts turned on him, right on queue: "Oh my god" "Are you serious?" It was obviously designed to catch him off-guard and it worked. That's the game, though (or at least it used to be), and ultimately he was a bad candidate because he wasn't prepared that kind of stuff.