From the source:
 
Applejack is the only one of the Mane Six whom I’d trust with my life. There is a lot of confidence, and safety that her character transmits to me. She’s reliable, loyal, truthful, and knows what she’s doing. When I saw her working at the farm, preparing rations for the army during the “Crystal War” Timeline, I thought to myself: “She’s going to be there for just a few weeks, before she puts on one of those suits and goes to the battlefield”. That’s why I drew this. To me Applejack is a mare that thrives in the heat of the battle as she does while bucking apples.