The fire inside is sucking in fresh air from the bottom door and exhausting it through the top window. Pretty basic convection pattern.

Candles. They have a tradition of doing something with candles (can't remember what) so there's many naked flames in the churches at that time. Happens every year around the same time.

The black smoke implies some sort of oil/petroleum fire. It's quite possible there was an oil-paint mural on the ceiling of the chapel.

I would assume that the heat of the fire below is pushing air up, while the burning ceiling pushes air back down, and the high pressure is just pushing all the gasses out the nearest exhaust port, in this case the highest window. I would assume the entire outside of the structure is stone, basically turning the entire building into a blast furnace.

But hey, maybe satan was the arsonist.