These are a lot of good points. I wonder what episode made up people's minds on A.J being the voice of reason? Anyways, allow me to explain more on some of my points.
^ Twilight has gotten angry at Applejack. In Applebuck Season, she was so frustrated at A.J that she was at the verge of shouting, and would have done so if A.J hadn't finally given in. In the season 2 premier, Twilight actually physically attacked Applejack, than latter just plain tackled her. In A Canterlot Wedding, Twilight shoved the hat, A.J so loved, right up her muzzle. Now, I'm one of those that believes that A.J and Twilight are closer friends, but A.J has pissed off Twi, and Twi her.
^ I'm not exaggerating. While, Fluttershy in Putting Your Hoove Down is probably one of the biggest fight scenes for any one mane 6 pony, Applejack has affected everyone in the gang the most.
The Last Round Up - No one has ever being able to truly get under Pinkie's skin and get her that angry (not even Gilda's actions). True, there's Party of One, but that was group effort. Heck, to a lesser extent, Rainbow was also upset at A.J here, enough so to take the lead from Twilight in planing, continually chase A.J after two of her friends fell, try to knock A.J off the carriage, and tackle her full force.
Twilight, I've already explained above, Rarity had a own episode of fighting with A.J and R.D argues with A.J all the time.
While, many can say, Applejack is the rock of the group, she also affects them the most with her actions. Can A.J still be reasonable in their eyes after pulling at their emotional strings so much?