She didn't say directly but on Ellen's tunjuk she berkata it was becoming hard keeping her long hair nd also trying new hairstyles,looks with that hair,idk actually
In another sumber I heard that she did it for a cancer campaign...
Well,but now the fact is she really wants her long hair back..she's trying really hard to grow it
She berkata it was because when she was working as Hannah Montana she had to look a certain way because she was under contract. She didn't have the option of changing her looks so when she did she decided to do the extreme opposite. Take a look here.