Claire, the military instills some of the symptoms of PTSD in all soldiers during training as they are requisites for life saving skills, ie. hypervigilance, alertness, awareness, etc etc.
I'd be inclined to agree, heightened situational awareness and constant vigilance is 'learned' behavior in this context. Training which is predicated upon people heading toward danger rather than away from it is another learned behavior, as is training which requires people to continue working until the job is done, regardless of what happens at different points in the process is also going to predict that those who do well in such training tend to 'cope' better than others, for longer than others, when things go badly wrong. This extended exposure, in my experience at least, coupled with effective response, has led to my PTSD being as severe as it is.
Put very simply, if I hadn't reacted so well at the time, I wouldn't have been in a position to have to react that way again (I'd have removed myself from the situation prior to the recurrence of danger). I reacted basically as I'd been trained to react, different context, but same 'type' of reaction.
Then again, I would have to say that incidents from childhood had a range of impacts, they wouldn't be classed as symptomatic of PTSD until after several extra incidents and several years of additional traumas.