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.
Laughs in Sweden
This is just their extremely long-term revenge for Old Alvsborg (see this Polanball - https://www.reddit.com/r/polandball/comments/2lynuc/the_fort_of_%C3%A4lvsborg_not_our_proudest_moment/) 400 years later...