Just like a stock. If somebody big enough to move a market decides to sell a stock that you have been watching, the selling pressure will drive the asking price of the stock down. But savvy buyers quickly move back in, especially if the stock is sound. When Bush's contract dropped to $52 and change last night, it was selling below it's perceived value, and hence was fairly quickly bid back up.
Also of note is what is happening to Daschle's contract. The bid is 37 and dropping. That bodes well for Thune.
Also note that Bush's Florida contract has dropped, but the Ohio contract is up strongly.