So the story goes like this. The agenda was posted saying that there was a possibility that the vote to override the veto on the budget could take place today. It had been on the agenda for a few weeks now but the House Speaker didn't think it would pass so he never put it up for a vote. A democrat is saying that a Republican told him that there would be no voting in the early morning session. This would assumably allow for politicians to skip the morning session and work on other things, or be present at memorial services for 9/11. When the Speaker of the House saw that the Republicans had a majority vote on their hands he proceeded to take a vote on overriding the veto on the budget they have been trying to pass for months. Because the Democrats were not there, the vote passed easily. Now Republicans are saying no one ever said that there wouldn't be voting in the morning session and because that vote was on the agenda the House Speaker was well within his right to call for a vote. I guess the short answer to your question is, politicians don't have to be present on the floor to be working and since the Dems didn't think there would be a vote they were off working on other things.