Main transformers don't last as long as they used to

PG&E had a big transformer fire the other day. As I've said before, I love to follow transformer stories. The article states that these huge beasts were expected to last 40 years, but they rarely last 20. This one was only 9 years old! Before they just zip in a new one, at 2 million dollars a pop, they are investigating the probably cause of the explosion. Perhaps it is a bad foundation?