In that case. IMO
It is the repetitive verbalization and emotion connected to that verbalization in a group setting with those without PTSD that causes the "strain". It creates a stickiness to that trauma that gets passed on to others and is hard to stay clear of it without the proper training. This causes tension and a leeriness to engage with those with PTSD.

At any rate that is what I have observed in both my behavior and that of others with PTSD and those without. I learned and stopped talking about my issues for this reason. At the time I did not know that I had PTSD, but understood the power of repetitiveness because of career choices and my general confusion about understanding others that forced me to take a hard look at this in my 30's.