At a guess the car may have been stored for a long time and when the engine was stopped that valve was open. As I said that is a guess but there's now way to know why this happened.
Often people buy 911s because it's their dream car but don't understand or aren't prepared for the running costs or the reality of driving a vintage sports car. (my car is 31 so she's vintage) Then for whatever reason they're stored away. Occasional use can cause some unexpected problem.
I would replace the failed one and drive it... Enjoy your 911 and give it a good "Spanking" every now and then. They really are well built cars. And their reputation for being reliable and robust is well founded. The SC in particular are considered tough.
