Could the increase in PTSD be a result of decreased drinking?
Entirely unrelated.
PTSD is the result of unpleasant experiences that the brain does not have the opportunity to process into memories, usually because survival takes priority for an extended time.
We have more PTSD now because we are far better at keeping people debilitated by it on the battlefield from getting killed .
Read some studies on how many cases there are believed to have been in the immediate wake of the civil war, and how those men were treated.