Why?

Because its typical internet usage is an exaggeration.

It is wrong to call every form of sadness a "depression", every form of aversion a "trigger", every lack of social skills an "autism", etc.

is the word trigger itself a trigger?

No, unless there is some weird form of torture when the prisoners are shown a word "trigger" written on screen, immediately followed by pain. So that even months or years after their torture ended, when they see the word "trigger", their hearts automatically starts beating fast, and they crouch and scream incontrollably.

Mere "I am so annoyed when I see someone speaking about triggers" is not a trigger.