Applejack falls for her own trap, missing out on helping with (probably) the last Confluence while her parents were alive. She swears off child-like things and becomes determined to only act very grown up. She runs away to the city to learn how to act very grown up and mature. Later she tries to negotiate very grown up business deals to prove she’s very grown up and capable of running the farm.
Her parents pass away
Applejack becomes de facto Team Mom for everyone, and everything she does must be mature and responsible. Nearly every single Applejack plot revolves around her trying to be very grown up and mature and responsible: keeping Apple Bloom safe (can’t lose any more Apples), looking out for Granny Smith, safekeeping Ponyville (and all Equestria), organizing events big and small, and always, always trying to provide for her parents’ farm.
In Going to Seed, Applejack finally drops her defenses and allows herself to be child-like