Because many trainers lack significant education in psychology or other social sciences, and because both training and therapy have at least one common theme (helping people learn), it's not uncommon for trainers to be unable to draw the line between training (an area they may be competent in), and therapy (an area they are almost certainly not competent in).
This is dangerous both for learners, and for trainers and their organizations, since it opens up the possibility of damage to learners, and also law suits.
Trainers need to understand there are limits to their skills, first of all. Even in the most mundane training situations, situations can occur that are charged with emotion and for which most trainers are ill-prepared. Training can unintentionally surface anxieties, memories, phobias and trigger other emotional reactions in a small number of learners. That, of course can happen to anyone, and the obvious course of action is for trainers to realize their limitations, and refer the learner to a competent professional.
What is worse, though is when trainers design and deliver training that is intended to have an emotional impact and uses exercises to create emotionally charged learning. Many training techniques get pretty close to that line. For example, outdoor types of training or team building can trigger strong emotional reactions. Even an activity like role-playing can re-evoke strong emotions. Imagine the situation where role playing sexual harrassment scenarios is used in a therapeutic way, and evokes a strong emotional reaction on the part of a sexual harrassment target of victim.
Trainers simply are not equipped to deal with issues that belong in therapy. Neither should trainers "mess with people' heads" in ways that may provoke emotions, even for the sake of learning and increasing self-awareness. It's a professional responsibility and obligation. It's a legal one.