One day, Applejack is arguing with Strawberry Sunrise, when Strawberry Sunrise catches sight of Appleboom. She acts strangely interested in her.

Later, when Applejack is off making deliveries, she approaches Appleboom. Don't worry, it's not a foalcon.

We learn that SS is a member of the Red Mane league. Apparently red manes are rare, and dying out in Equestria. To prevent this from happening, the league was formed. They give social advantages to ponies with Red Mane hair, and ultimately help them find marriage partners who have either red Manes, or with whom the colour red will dominate. It's a secret society because Celestia doesn't want selective breeding eugenics to catch on for some completely unknown reason.

Will Applebloom keep the secret?