Mac has dedicated his life to the farm, to his sisters, and Applejack feels bad for it, she feels it's time for her brother to star having fun again to have a life beyond his work, friends and even more, but she isn't going to let him go just with any other mare, no, she need something good and harmless.
Then there's Rarity, ever since the coronation she's been work on multiple designs for multiple clients, and she hasn't seen much of her friends ever since either, or done any other thing that isn't working.

For Applejack it was a matter of putting one and one together. A flawless harmless plan.

(half way trough editing)