Part of the pathology of the Rifa cap is the nature of the plastic package. Another part is degradation of the film, where the metallic film on the paper is eaten away and they lose capacity over time. It might be some of the vapor from this process that expands the initial volume and starts to encourage the cracking of the plastic.
The plastic initially gets multiple fine cracks. This starts to let in air and water vapor. This is absorbed by the paper. Long before they smoke, they start to get volume expansion and swell up. This significantly opens the cracks and it is a positive feedback effect. Once they are swollen enough and the paper contains enough water they start to electrically conduct, presumably due to residual salts in the paper or salt products derived from the vaporized metal film creating a weak electrolyte, then they start to heat and burn up. In some cases you will see the manufacturer was aware of this and put a low wattage low Ohm value carbon resistor is series that will act as a kind of fuse when the capacitor starts to conduct heavily.
Descriptive terms applied to the Rifa include safety capacitor, fire resistant, self healing and other overly optimistic descriptors. They should never be used as spark suppressors across switch contacts in series with motors or industrial machinery (they often are) they can spontaneously short out starting the machine. I have told a story once how I was sleeping in a room with a Singer Sewing machine, the Rifa cap across the control pedal shorted out and the machine started at full speed in the middle of the night.
It is a combination of the paper dielectric and the type of plastic casing that causes the trouble.
On the other hand, capacitors using paper dielectric and a solid foil, immersed in oil, like the typical PIO capacitor in metal canister, doesn't have these issues and could be called fire proof too because of the metal encapsulation. But for the same uF value and AC & DC voltage rating, these are much bigger than a Rifa. This is the price to pay when you put miniaturization in front of reliability and safety.
