Image from acquerra.com.au (I couldn't get permission, no contact info on website)
This has happened before. Jupiter's southern belt seems to disappear on a roughly 15-year cycle, with the last disappearance being in the early 1990s. Astronomers weren't really quite sure what was going on. Amateur astronomer Anthony Wesley from Australia was the first to catch a glimpse of the belt's disappearance as it rounded the sun.
"It is not known why the belt periodically disappears, but it may be that it sinks lower if it cools, and then the view of it is obscured by clouds pouring in over the top of it. The clouds on Jupiter are tens of thousands of kilometers deep. Wesley said the phenomenon could be linked to storm activity that preceded the change." (Jupiter has lost one of its cloud stripes - Physorg.com)
Wow. Apparently, astronomers were suspecting that Jupiter would be going through another fading cycle as it passed behind the sun. The exact date probably never will be discovered.