Saw a documentary a few years ago where they reconstrucred how the ramparts may have become 'vitrified. The conclusion was that the extreme temperatures which melted the rock were reached because the forts were in fairly exposed areas where high winds were common, acting a little like a bellows i suppose. Not sure why the whole of the ramparts are fused, can only guess that the timber used to build the walkways common on many hillforts became well seasoned when old and dry and was possibly resinous (scotts pine?) making it condusive to being flammable. Only a theory though.
Couldn't have been the Romans that did it though, as they never reached into the far North of Scotland, or so were told. Maybe the attackers threw torches coated in resin or some tar like bitumen substance.