As Leah said, there is a difference between challenging someone or pushing them vs purposefully triggering them.

I think part of it depends on the therapist's training and also their competence, not to mention intention. A therapist might trigger you because they are incompetent. They might trigger you because the particular type of therapy is more confrontational (e.g. REBT vs humanistic). They might trigger you cause they are mean or unethical (e.g. racist or sexist or whatever). They might trigger you because they don't know you well enough (assuming it's quite early in your meetings) and accidentally touch on hot button issue.

A good therapist is careful. Like a good teacher, they push you forward to face your problems. I think it's best (and many therapists don't do this) to review the purpose of therapy with the client regularly. What are the things you are trying to improve or fix or get past. With that clear in mind, you and therapist will be working together to move forward towards them. But you can't get there if you never feel uncomfortable. Some level of discomfort is necessary. The important part is that it's not too much or too sudden or trigger someone badly. Getting triggered is inevitable in therapy. It will happen for everyone. That's different from getting triggered in a big way or getting repeatedly triggered.

But all this is in service of improving the person's mental health. So if I go to my therapist and say I wish my mom was dead, there is a lot of anger there. Pretty much a lot of triggers. It's a minefield. If my purpose is to improve my relationships, we may have to get back to this one at some point. And the therapist will need to push me to express certain feelings I am not comfortable with. For instance in reality I am very uncomfortable with expressing anger or being exposed to anger. It's almost always triggering for me. My therapist had to push me to express some of those feelings and I ended up angry at her! At some point I saw her like mom and felt infuriated at her. But she realized in time to back off. She learned how much to push me. She pushed me...and then let go...pushed me...and let go, giving me time to recover and see where I'm at and how I'm doing. It's a careful process. Both sides have to feel like they're on the same page. It takes skill.