The bulding was undergoing a renovation.
Old building + hot work = frequent fires.
This is how Notre Dam burned. It's how the old Capitol building in Iowa burned. It's how the Yonkers building burned. If you don't like it, the answer isn't to blame a secret cabal that wants to destroy history. THE ANSWER IS TO STOP IMPORTING FUCKING INCOMPETENT IMMIGRANT CONSTRUCTION WORKERS.
........
Okay on further investigation it appears Notre Dame was not hot work (although it was occurring, just not on that day) but rather the scaffolding workers were smoking. The investigators found cigarette butts but couldn't conclusively prove they were the ignition source.
Damn sure is a good thing western governments are trustworthy and never lie.