I assume that you are talking about the part at the end of Chapter 4. Here, Gene is talking about what happened when he caused Finny to fall off the tree branch and get so badly hurt.
Writing teachers will tell you that there is a real difference in meaning and feel when you say something in the passive tense like this. When you say it in the passive, it is like something just happened without anyone actually doing it. That's what Gene is doing here.
If he said "I bent my knees" then he is acting and its his fault. If he says "my knees bent" then it's like they just accidentally bent -- he didn't do it and it's not his fault.