Why did Shouko change her hair style? This is the first time Shouko change her hair style to ponytail. What's the reason ?
I find the answer in Volume 4, chapter 24 ( read from right to left or look at the page number )

Volume 4, chapter 24, page 6,7,8



Ishida realized that what Kawaii do to her appearance is clearly the same with Shouko at chapter 23. It's pretty obvious that Kawaii like Mashiba. 

So The reason why Kawaii change her appearance is because she want to look attractive in front of Mashiba. It's same with Shouko case. 

It's just my speculation but from the interpreter of the story at chapter 23 said that Shouko want to look attractive ( she want to looks different ) in front of Ishida, and after that he confession to Ishida but it didn't go so well. But Ishida still didn't know what shouko feel toward him.