Probably lit it themselves.

It's a historic building, they built a new one, probably couldn't get a demolition permit for the old one, but if it burns...

I've seen a church near me do the same sort of thing. An old house that they wanted to tear down 'happened' to catch on fire. No question it was arson, the electricity was turned off, the fire started in the ceiling. I thwarted their plans, I was working nearby, saw smoke, and grabbed a fire extinguisher. Got it out before the fire department got there.