That's a really good question. I suffer with PTSD and only my close friends know about my diagnosis. All of those close friends believe that I have it due to a grief reaction when my parents died suddenly. Same for my employer.
Only my sister, my GP/psychiatrist/therapist and my DH know that the grief just made my PTSD symptoms impossible to live with and pushed me to go to the doctor about it.
My close friends are lovely and just reacted to my diagnosis with understanding, and said 'oh yes, I was the same after my mum died' (with the best intentions, and I love them for it).
When I talk about what happened to me sometimes I dissociate, sometimes I seem totally fine and but then end up dealing with suicidal impulses a few days later, sometimes I get flashbacks or can't sleep for 72 hours at a stretch etc. etc.
I won't tell you why I have PTSD. Like I say, I wouldn't tell my closest friends and if I wrote about it here now I wouldn't be able to work or function for the rest of the day. But I need something that I can say to a friend over coffee, and my parents' death is a reasonable and socially acceptable thing to say, so I say that.
Can I gently suggest to anyone reading this, that if someone tells you they have PTSD, maybe the best follow up question isn't "What did you get that from?" but "Is there anything I can do that would help you right now?" You can just go ahead and assume that if someone has PTSD then something happened that they weren't able to process, and leave it at that, unless they actually want to talk to you about it.
Maybe with all the 'destigmatise mental health' movement these days, there's an idea that being really open about everything is good. But for my own case, when it comes to talking about trauma it is best done in very controlled and safe circumstances, not necessarily in the HR office of a company or in Starbucks with a friend.
Just to reiterate - it's a really good question, I'm glad that you asked it OP, because I think it would be good for more people to understand this.