On the one hand if most of these forts are in Scotland, it could mean the plasma events were more common in that area or that there just was more of these forts in Scotland at the time.
It is also interesting to note that when the Sarmatian Dal Riada clan invaded Ireland, they claimed the Irish natives could control the weather, according to the surviving legends.
I'm thinking it had to have been done deliberately by people - if plasma was the cause, shouldn't there be green glass all over Scotland and not just the occasional vitrified stone fort?