Because at this moment something from the unconscious slips into consciousness, some kind of forbidden thought. Then it is repressed into the unconscious along with other thoughts that at that moment were relevant in consciousness. There remains a void. Interestingly, the most popular way to solve this situation is to go back and repeat the process again. We are trying to restore the logic of thinking in order to get the very thought out of oblivion.
Why are we doing this? Because this is how the unconscious works. After all, this amnesia does not mean that thoughts have disappeared and cannot be restored. Thoughts were repressed into the unconscious and are now there. When we return to the same place, we try, along the chains of associations - smells, sensations, the type of room - to restore the contents of our mind at that moment.
Freud does exactly the same when he deals with the interpretation of dreams - through chains of associations he returns to the real meaning of a dream. Amazing thing. We all use psychoanalysis without knowing we are using it. Here it is, unconscious knowledge)