Could it have been an insulin pump? Perhaps Bush was using one to treat diabetes brought on by days of heavy drinking, this letter to Salon suggests (scroll down to read the letter). Could it have been some other medical device, like a portable defibrillator that could prevent cardiac arrest? Some sites, including this one (Theory No. 2), point out that Bush has a family history of irregular heartbeats.
Or maybe the president has had a stroke, and the boxlike bulge on his back was a machine dispensing medication. In this thread on DailyKos, posters debate the merits of this far-fetched scenario. "Check Bush's mouth, where the spittle was coming out," the first post reads. "It's slightly droopy. If you go back and look at video from his earlier days, his mouth isn't drooping, that side of his face is far more animated."