Ah, Dave, the short answer seems to be "no one has a clue," but read on for the guesses I've heard about.

First, you might visit straightdope.com, a very entertaining site that addresses imponderables such as this. The author, Cecil Adams, has addressed this one several times, so you might get more from him.

The leading candidates seem to be:

Something to do with football. Still, you gotta get 10 yards to make a first down, so this connection is pretty tenuous.

Something to do with the clothing industry. Various theories point out that 9 yards of fabric would make a swell suit; others say that that is way too much. I'm no tailor so I have no idea.

Something to do with ammunition. If you can picture those Mexican federales in "Treasure of the Sierra Madre," you have it. Those big loopy ammunition straps (whose name escapes me) may have been 9 yards long total, but then again, maybe not.

Something to do with WWII. Again, my memory of this is vague, but rest assured, it's another big fat stretch of the imagination.

So again, nobody knows, but there's no shortage of theories. Anyone else?