Hi UnforgivenII
Great question.
"triggered" -- having non-constructive reactions to specific words or actions based on prior experiences. We've all been there - resentment, pessimism, defensiveness, impatience, closed mindedness, distrusting, intolerance, confrontational, defeated... .
There's a concise description for you. I encourage you to see the rest of that post as it has some helpful tips.
Example: When you tell a pwBPD that they are wrong in some way (action). That can be a simple trigger for hurt (reaction), which may not be non-constructive as it helps one to know that an apology / adjusting action may be in order. But, the pwBPD can also react from the same action by a series of non-constructive ways, such as excessive pain (reaction) or excessive defensiveness (reaction).
Not all triggers are "bad".