As to why it happened, in the postop section I commented in a thread about soul-source dilators.
In that comment I included some descriptions of some of the challenges faced by a surgeon in constructing a neovagina, and indeed in avoiding this very issue. The problem is lack of the normal suspensory ligaments attached to the uterus.
You might find it interesting to read.
Correct dilation technique is important during the early stages, but to be honest complications do happen, and often it is no ones fault, just bad luck. It happened to my ex partner. She got a Resistant Staphylococcus Aureus infection in the wound and she lost almost all her vaginal depth. That was back in the 1980's - so these things happen and the important thing is to get it fixed - which can be done.