would \NOT\ attribute to terrorism...
Gotcha - yes, I agree. Probably not terrorism.
I would not expect that the transformer sat idle all winter, they’re usually in service unless out for maintenance (which is possible). A multitude of things could go wrong, from internal failure due to defect to overheating and failure to respond and reduce loading in a timely fashion to protect the equipment.
The fire is not unexpected in an actual internal fault event - it can explode pretty violently inside and will generally rupture the outer tank (thick steel) at weld points, bolted connections, anything that is weaker than the rest of the case. Fire is likely though once in a while, they just rupture and spew hot oil around the transformer. No doubt it tripped off line, as it should, to prevent expanding the area of damage and feeding the faulted equipment.