John's childhood journal details a past life locked within his subconscious. As he reads, his nightmare projects to his reality amongst his vision, detailing the power of an event so intense that I can only describe as violently jarring one's psyche.
However, constant adrenaline, placing the lives of your team first, and the implementation of training to react and think simultaneously creates so many fight or flight triggers. Over the course of an extended timeframe this may cause the triggers in the future to become more pronounced, elevating their severity.
The scene represents how life events may take over the present reality, even when they occured in the past. John's recent experience from the first chapter is evolving past his dreams and into his reality.