If you have a phobia, one natural reaction is to anticipate in which situations the "trigger" will occur and to actively avoid such situations. It's well-known in psychology however that this behavior actually reinforces the phobia and prevents it from healing.

Trigger warnings seem like the same strategy only elevated on an institutional level. So while they do prevent panic attacks, they would actually do the traumatized more harm than good.

The other thing is that people can develop phobias against almost everything. So no matter how many trigger warnings the university put up, there could always be one group which finds their particular triggers not marked up well enough.