"Most archaeologists now consider that vitrified forts are the product of deliberate destruction either following the capture of the site by an enemy force or by the occupants at the end of its active life as an act of ritual closure"
I suspect the real answer is that nobody really has any idea how or why this was done - which makes it all the more fascinating :-)
Personally, I think vitrification was a product of accident/attack rather than direct intent...