MMM. There's a couple of spanners in the works:
1. Wincobank, which I think is a vitrified fort the interior area is about average for a hillfort and would cook the chaps inside if the walls were at 1100 degrees.
2. All hill forts have an outer ditch would be a perfect place to put fires yet most hillforts have no signs of this being done. Don't forget most of the defences relied on wood for their structure, there are many cases where wooden parts were covered with leather to protect against fire.
3. In the case of Almondbury, (which I think is a dodgy excavation conclusion) the fort was deserted after vitrification.