I don't think it's a silly question at all Chris. But I don't know the answer - if they had been leaky I could have understood overheating and burning, but not otherwise.
I'm all for encouraging murphyv310 and twocvbloke to send failure reports to Rifa (part of Kemet these days) - the PME271 range is still in production and field failures do provide reputable companies the opportunity to improve their products, and design-out the failure modes in future designs.
Here is some information on these capacitors. Interestingly they reckon that metallised paper is superior to metallised plastic for repetitive transients.