I can give you tons of examples but the explanation is a simple one: If the client reactivates emotions and/or memories by feeling them as they happened during the traumatization, and the therapist misses to intervene in ways that the client could:
tolerate the affect, or
find a way to contain it, or
reconsolidates the memory changing the valence from negative to positive.
If that doesn’t happen, the client will probably end up retraumatized and it’d be the responsibility of the therapist since s[h]e allowed the activation without the resolution or containment. Most therapists didn't know that, and most of them have been trying to provoke catharsis thinking that it is beneficial. Therapists that were trained a long time ago that have not actualized themselves still work too often in traumatic memories without any type of preparation. Without regulation skills and resources, working on the past could be more detrimental than it’s beneficial.
But what exactly is retraumatization? Maybe people are using the term without understanding its severity.
Traumatization is to activate the survival mechanism in order to protect the system from perishing because the brain assumes that it’s the only way given the perceived danger. Those mechanisms create many alterations in the system that cause symptoms to manifest. Trauma happens when those mechanisms keep active because the brain assumes that the danger is still present (by continuing beating afraid) and when the symptoms continue. In general, the traumatized person can become less scared, and the survival mode fades away, or the activation of the mechanisms becomes chronic within a range.
Retraumatization happens when the person’s brain perceives danger again, the same type that traumatized him/her, and reactivates the survival mechanisms more extremely to avoid perishing. Since the brain doesn’t recognize real from imaginary, by the client remembering the traumatic events and feeling as scared as before, the brain assumes that it needs to act more extremely in order to maintain the person alive, since the previous time it almost failed (that's why it stayed in survival mode). Therefore, retraumatization is not only activating the defenses again but to activate them more extremely. It implies that the person will get worse and the symptoms worsen.