If this was arson (and at this point there is no evidence of it), I would think extremist environmentalist groups would be the likely suspects. Or some disgruntled employee.
I find it really, really hard to believe that a developer would do such a thing. If the project fails, the bank forecloses and they lose money. Big deal. But arson and insurance fraud convictions would result in losing everything they have and also going to jail for a long, long time. Nobody believes that insurance companies would just roll over and pay without thoroughly investigating the cause. It just doesn’t make any sense for the developer to do anything like that.