Only a fraction of persons who experience trauma will actually go on to have "abnormal" responses to trauma that require psychological intervention. Most people that experience trauma will understandably go through some distress but are actually able to go on to reconcile their experience by themselves or with the help of their family, friends and community. Thus forcing people to debrief directly after a trauma may disrupt "normal" processing of the event. What is also particularly disconcerting about the research findings on trauma debriefings is that many people interviewed about their experience of trauma debriefing expressed they found the trauma debriefing helpful. However, research actually indicates that the debriefing is still more detrimental than no debriefing despite what the trauma survivor may perceive.
The following article goes into some more detail about this and specifically talks of the dangers of well-intentioned trauma debriefings in the contexts of disaster relief (such as the 2004 Asian tsunami and the more recent events in Boston, Dhaka, Syria and Mali): Minds traumatised by disaster heal themselves without therapy
It is important to note that some survivors of trauma will experience particular difficulties and symptoms that may require psychological intervention. These symptoms are likely to include persistent re-experiencing of the event, avoidance of stimuli associated with the trauma and increased arousal (difficulties sleep, anger, concentration and hyper-vigilance). These symptoms persist for more than several weeks and cause significant distress, and impairment in social and occupational functioning. There are various psychological interventions - that are not debriefings directly after the trauma - that have been shown to help with these difficulties.