Then why didn't it show up during unit assembly or testing??
[However it could also be chaffing/fretting of wire to expose conductors in a longer timescale, but I find it hard to believe an ELT Tech working at a bench in the factory wouldn't spot this during assembly]
I've been there and you can bet your life those questions, and then some, are being asked right now!
However a pinched wire would not necessarily be visible from an external inspection and could well not have created a short straight away. My guess is the combination of a pinched wire followed by the temperature cycling eventually ruptured the wire insulation and if, as I speculated earlier, the fact the wire was pinched also compromised the hermetic seal and allowed the damp in, well - there you go!