I thought the point of a trigger warning was to enable people with PTSD or anxiety disorders to leave the room or avoid the material rather than sit though it and feel worse.

"“the point is not to enable—let alone encourage—students to skip these readings or our subsequent class discussion (both of which are mandatory in my courses, absent a formal exemption). Rather, it is to allow those who are sensitive to these subjects to prepare themselves for reading about them, and better manage their reactions.” "

I feel that this professor has completely reversed the material benefit of it. If you say this could cause you severe anxiety but I'm going to make you sit though it anyway what was the point? It doesn't surprise me that if you trap the audience and warn them it will make them feel worse.