How in the world does 1 tire go 1/2 flat?

As some of you already know, I went to the Los Magnificos show in Houston this weekend. Anyhow, it's about a good 2.5-3 hours from Austin, door-to-door.

I got 130 miles into the trip, doing about 80 (yes, in an 80 ) on the highway. I stopped at a truck stop on I-10 to drain the weasel, and upon returning to my baby I noticed the rear-left wheel was partially deflated. Of course my first thought was that I got a flat, perhaps from the crappy parking lot of the truck stop. But the tire wasn't fully deflated.....so I thought maybe it had been slowly deflating during the ride. The tire didn't feel any warmer than the others, though, so I think it's pretty safe to assume I hadn't been driving highways speeds with an underinflated tire.

I reinflated the tire, drove around a little, and then went on my merry way. ~350 miles later of 70 mph driving (yes, I slowed down a little) and the tire seems good as it was before it lost the air.

So the question is....how in the hell does one tire lose most of its air when it can hold the air just fine afterwards???? Did I take the exit ramp turn a little too fast (~40)? Did I just happen to hit a small bump that robbed me of air? Is my tire and me not knowing it??

OH yeah....and

b