In the book, one major life-changing event appears to be the catalyst for Billy's non-linear time travels: his war-induced PTSD (Post Traumatic Stress Disorder).
We know that Billy's devastating experiences during and after the bombing of Dresden severely traumatized him. Even after he returns from the war, he experiences extreme fluctuations in emotion; he is often anxious and exhibits symptoms of hyper-arousal, which are commonly associated with PTSD sufferers. Billy's conception of time and place continually evolves, and his time travels appear to function as a clever metaphor for his dissociative disorder.
Like all PTSD sufferers, Billy often finds himself emotionally incapacitated by "triggers" that force him to relive his war-time experiences during the most inopportune moments. For example, during his and Valencia's anniversary party, Billy becomes discomposed when the barbershop quartet of optometrists sing "That Old Gang of Mine." The song triggers Billy, and he becomes...
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