Well, if we're going to speculate, here's my proposal:
Lady Sun was a spoiled brat. Sun Jian spent whatever time he had at home with his older sons, making sure that they're learning the right stuff and are going to be able to carry on his work, and with Lady Wu (hey, they did have at least 5 kids together, despite all his time being away and leaving his family in Shouchun). Lady Sun inherited all the fiery characteristics of Sun Jian but wasn't tempered by a stable family environment (they moved around a lot during Lady Sun's childhood: Xiapi [Jiangbiao Zhuan from Sun Quan's SGZ bio] -> Shouchun [Jiangbiao Zhuan from Sun Ce's bio] -> Shu [Sun Ce's bio; this is not the Shu in the Riverlands] -> Qu'e [after Sun Jian's death; Sun Ce's bio] -> Jiangdu [Sun Ce's bio]). Lady Wu was probably busy with arranging the moves, taking care of the clan, etc, and didn't have as much time to take care of Lady Sun as she would have if she were a housewife staying in a stable household. Being of the same temperament as her father and her brothers Sun Ce and Sun Yi, Lady Sun wanted to do all the boys stuff, and there's no one to stop her. Thus she grew up a tomboy and terrorized the family. She had her maids trained in the martial arts because her brothers wouldn't play fight with her; this army of women also became a formidable presence in the Sun household.
When Liu Bei became a considerable force, Sun Quan wanted to make him his ally. He was also worried about his sister being entirely unmarriageable. So bingo!, he thought. Two birds with one stone! I'll marry my sister to Liu Bei, and Liu Bei would be family to me (whether he loves her or not), and I'll be rid of this brat! Thus the famous wedding took place, and Lady Sun was sent back with Liu Bei. However, since she was so arrogant and unrestrained [Zhao Yun Bie Zhuan
, Zhao Yun's SGZ bio], and her maids so terrifying, Liu Bei would have nothing to do with her and left her in Jingzhou, guarded by Zhao Yun, while he went to do nasty work in the Riverlands.
This is my analysis from the history... in the novel, where she's actually a nice person, things may be different