Why did Yu fall?

I got lost in the turmoil after my death. Yu Zhi Ding is a short story by Wu Cheng'en in Ming Dynasty. Refers to books such as Youyang Zaju and Xuanguailu. As stated in the preface, he loved the biographies of Zhang Qi and Duan Kegu, and was good at writing things and feelings. Every time he wants to write a book about it, he is too lazy to take the time. He had more than a dozen original books, but he lost them after his death. Now he can see the original preface.