What happened to Brian when he was younger that still impacts him?
Brian learning of "the secret" is something that looms large over him. Despite the fact that he is fighting for survival, he is unable to escape "the secret." This is what casts a shadow over him. When he was riding bikes with his friend Terry, Brian saw his mother with another man. It is because of this, something that he has not said to his father, that his mother wanted the divorce that causes him to fly out to his father's in the first place. "The secret" is what he cannot shake and is what is looming over him.
The exact time as to when this happened is not told to us. Given the fact that the story opens with Brian getting on the plane and the divorce being relatively recent, one can assume that Brian saw his mother in the parking lot with the other man within the past year. Brian has not been able to shake the memory and it is for this reason that even while fighting for survival, Brian still struggles with the memory of what he saw and what it meant for his life.