I guess at some level that child part of you suddenly decided this situtaion was safe for whatever reason. All dissociation is a safety mechanism; the things that happened were too overwhelming, so the trauma is stored 'separated' from 'you'. It stays that way until there is a safe place/person/way to deal with it. As an adult I switch parts if I'm under threat (much to my horror at times) - but it is my minds way of trying to find a part of me that will cope with the situation I'm in. It is rather maladaptive for me now, but it saved me when I was small.
You may or may not ever figure out why this T, at this time was the 'right' place to be able to begin to process the trauma - but it is a good sign (if uncomfortable).
Hope those thoughts help a bit?
SB