Hard to say since Chris was likely lied to about everything that transpired with that incident, and by the time of the Father Call, it was the end all be all defining catastrophe of his childhood.
My guess is Chris didn't change clothes very often as a kid, and eventually developed a rash on his neck where the dirty collar was irritating his skin.
Over time, he conflated that rash with being caused by the incident, because he grew up being told that incident was the source of all his educational troubles, when it was probably what triggered the school to take notice that something about him wasn't right.
Remember Chris believes that the people involved that day went as far as to manipulate the real estate market against his parents to keep them from moving or something. He's insane.