Understanding the societal impact of PTSD is important because we do have so many people in our country, as well as, other countries who are suffering from it. It does come with a certain level of necessary social awareness and responsibility.
People with PTSD don't generally wear t-shirts stating their condition, nor do they announce their PTSD in public outings, or even in the workplace. They carry it quietly with them, trying to mask what it really feels like on the inside. When someone approaches someone with PTSD from behind and tugs on their shirt, pokes them, or slaps them on the back it sends the brain into the overdrive mode of fight, flight, or freeze. It is not the normal startle of someone who was just caught off guard, it is an event that takes some considerable effort to counteract. The brain doesn't just "relax" immediately because you see it's just "so and so" from the office or it's just your neighbor saying hello. The jarring response stays present, creates a raised heart rate, spikes cortisol levels, and well, can create a situation of anger and unrest. Overall, it is a very unpleasant and upsetting situation to be in, when it could have been easily avoided had the person simply announced their presence and approached you from the front.
This is where some social responsibility can be exercised. Our current, former, and retired service members, police officers, firefighters, and civilians who have experienced trauma and may very well have a PTSD diagnosis, are embedded in our society. You aren't always going to know who they are, but there are a lot of them. It's best not to approach people from behind without announcing yourself first and definitely, do not go for the pat on the back prior to announcing yourself. They need to know you are there, first and foremost, before those kinds of interactions take place.
I talk a bit about this in meditation sessions with those I teach. I always let them know if I will be standing up or moving during the session, so that they do not become alarmed. I also let them know that if they do happen to fall asleep, I will not touch them to wake them. I have other cues that I use to help with that.
What may seem innocent to the instructor, may be very frightening or jarring to the student. It's important that we all understand that the element of surprise is never okay for someone with PTSD. It takes time to develop the skills to "roll" with the unexpected and even then, it can present some pretty distinct challenges.