Wow, how is it even possible to 'hold it in'; I mean at one point wouldn't' you uncontrollably just go or in your sleep
This may be a bit TMI, but the longer faeces is in the colon, the drier it becomes. Eventually it can become so dry and inflexible that it can no longer be passed through normal means and causes an obstruction, backing everything else up. In severe cases like this one, organs are displaced by the impaction, and pressure on the heart impedes its function.
This girl was autistic - I wouldn't be surprised if that contributed to her death. Really tragic.