I received this privately from a member of this board today and it is too good not to share with you all:

The expression has been around fliers and Air Force types for ages. I heard it in WWII and my Mother told me she had heard it in days of barnstormers (1920s.)

I've heard a lot of tales but the one that seems to make the most COMMON sense is that in early days of flight planes often crashed. Pilots usually tried to avoid populated areas and put down planes or crashed generally on farmlands. How it connects I don't know or have forgotten.

It will be interesting hear what others ( of a generation or 2 or 3 younger) have to offer!