This is mentioned in Chapter 5, when Holden writes a composition based on his brother's mitt that leads into a very revealing story about what happened in Holden's past and what probably is still troubling him in the present. Holden tells us about his younger brother Allie and how he died from leukemia. Holden clearly has issues with his grief, as he records that after his death Holden hurt himself very badly:
I was only thirteen, and they were going to have me psychoanalysed and all, because I broke all the windows in the garage. I don't blame them.
Holden reports that his "hand was already broken" after doing this so he couldn't break the windows on the station wagon they had. Clearly this is a massively important issue for Holden as he harmed himself and was sent of for psychoanalysis. We can trace the breakdown that he experiences in this book to his brother's early death and his inability to cope with it.