I too think it was intentionally lit, but you seem to have forgotten the purpose of a roof: it’s to keep everything underneath it DRY!
When fire starts in dry areas, it spreads through the dry areas, weakening the structure until collapse. This structure has been drying for 600 years.
So, even if there was a foot of snow on the roof, when a fire starts inside, it’s going to burn hot and fast unless extinguished.
Look what I just found...This is an article from last month.. They don’t know why? Are they serious?