Exaggerated startle response is also common with various types of anxiety and panic disorders. I've had one my whole life and only figured out why after being diagnosed with generalized anxiety as an adult. Though Bruce's issues are clearly related to PTSD, they look an awful lot like anxiety/panic sometimes.

FYI, though I did not comment separately, I read and enjoyed the Schrodinger's Heroes/Avengers crossovers this past weekend.