It happens. It happened in a high school class I was teaching once. Out of the blue my student just started crying. I tried to ask her what the problem was, but it made it worse. I later found that it was a PTSD issue, but I never did find out exactly what triggered it (and I don't remember exactly what we were talking about). The student eventually had to drop my course. She wrote me a very nice letter apologising profusely and saying that she thought I was a good teacher, but that she couldn't continue with the class.

Having said that, I'm not sure a trigger warning is necessarily a good idea. It's a pretty serious situation and I don't think that you could come up with a blanket solution that would work.

That's like saying "What do we need wheel chair ramps for?" I've never seen a person in a wheel chair get stuck trying to access the building. Partially disabled people are a minority, partially people don't pay that much attention, and partially people are self-selected out of such situations. Panic attacks are a disability. Trigger warners are not to censor, but to INCREASE the audience participation. If anything, it's anti-censorship.