Whilst doing my A-Level donkey's ago many of the excavation reports for iron age defended hilltops stated that the majority of domestic habitation was situated close to the inner rampart, which could account for your timber ash.
Is there any evidence that the vitrification process wasn't caused by fire, but by some chemical reaction? Some chemicals are quite capable of 'melting' stone.
On a slightly different track, if you had a chemical coctail of stuff like magnesium, phosphorus or pottasium, then high enough temperatures may have been attainable to melt the rock (though i suppose this would depend on the rock type, not a geologist so that's out of my area of understanding).