Apple Bloom learnt young that a good cry and a pout and pleading with her eyes would bring forgiveness and treats. Everyone doted on her (except for Miss Rarity from the village, but she was awful with children and would do anything to stop Apple Bloom crying), and she never wanted for anything.
Except friends. Apple Bloom wanted friends. When she was six, she got friends. She liked Scootaloo more than Sweetie Belle; Sweetie Belle was a bit too calm and safe for Apple Bloom's liking, but Scootaloo liked to jump into things.
For a while, Apple Bloom was very happy. But when she turned thirteen, she discovered that teenagers didn't get away with crying. Apple Bloom was many things, but stupid she was not; when tears didn't work, she threw her pink bow in the bin and wiped the pout off her face and became an angry teenager.
AB was a very angry teenager, desperate to pass on blame. AB believed Scootaloo over everything, even when Sweetie Belle ran away. It was all Sweetie Belle's fault.
But angry teenagers aren't always right.