In Part One, Hinata cuts her hair short and it makes her look hair look like a duck's tail (sorry, Hinata). She lets it grow long in Part Two, the cuts it short again in Part Three. There may not be any reason why she does so, but can someone tell me why Hinata cuts her hair short (if there even is any reason)?
I heard someone say that Sasuke likes girls with long hair. Because Hinata has a crush on Naruto, she wants to keep her hair short so that Sasuke won't like her, is he ever does.
The other reason is that the tradition in Japan is that girls have their hair cut short if they are a disgrace to their clan/family. Hinata was not very strong at Part One, as she was defeated by her little sister Hanabi, though she was 5 years younger. Hiyashi Hyuga, Hinata's dad, even referred to her as a disgrace (in one of the filler episodes).