The reason of course is the all night sitting which Senator Reid has organised to discuss the Levin-Reid amendment to withdraw US troops from Iraq. In order to highlight the filibustering by the Republicans, a thirty hour debate is being conducted. Democrats want a vote on the amendment, which they would probably win - as some Republicans are backing the amendment. But while the Republican leadership is backing filibustering - 60 votes (as opposed to 51) are needed to end debate. Hence the public protest.
Senate Majority Whip, Richard Durbin said - "It's time to really make that point that if the Republicans are going to filibuster and try to stop the debate on the War in Iraq, then frankly, they have to stay here and pay the price," said Durbin. "Too many times we've sanitized these filibusters so that members can go out to a nice dinner, go home, go to sleep, get up in the morning, and say 'well, the filibuster's over.'"
There's an interesting take on the mechanics of an all night sitting - the need for mattresses for individual Senators to take a nap at http://www.washingtonpost.com/wp-dyn/content/article/2007/07/17/AR2007071702163.html?hpid%3Dartslot&sub=AR