Well, that's my first guess as well.
I told the wife about it and she turned on FOX, where its the big live story. Shep had to get in that "anyone who is leading with conspiracy theories is way off base at this point." Please.
The reporter making the speculation that this was arson, was in Paris reporting on the fire. Shep shut him down and ended the conversation. I had the arson thoughts myself. Notre Dame Cathedral was being restored by workers than many could have been Muslim. It would be easy to start a fire high up, while on a scaffold, just as the workers left the building at the end of the shift. The timing of the fire, just as Holy Week commenced, is too coincidental.