transformers are filled with flammable oil, which is so similar to diesel some people even use it as such. moderate physical damage will cause short circuit, and this will provide enough energy to disperse and ignite that oil. no need for pressure buildup from fire

Can transformer cooling oil form flammable vapour? Maybe, I guess, if it's hot enough, but I'm not sure. But when the transformer gets hot and explodes it may cause an oil spray fine enough to create a fireball, which may look similar. The first stage of a "proper" BLEVE is normally the "expanding vapor" cloud, which is visible as such, before it has mixed with air sufficiently (and/or reaches an ignition source) to burn and form the fireball. Then again, in smaller ones, and in the dark, the vapour cloud may be so short-lived that it cannot be seen.