I'm pretty sure we know the answer to that one!
Here is a link to the Fairbanks News-Miner that explains the situation. It's somewhat complex and still up the air to some degree. But it appears that the Alaskan senators didn't understand the complex funding provisions contained in the bill. It turns out that the earmarked funding for the two bridges was going to come out of $1B in pork already going to Alaska, so now they want to remove the bridge earmarks and just keep the money!
http://www.news-miner.com/Stories/0,1413,113~7244~3131972,00.html
Also interesting that Sierra's complaints regarding conflict of interest by AK Sen. Murkowski because her family owns land on the island where one of the bridges was to be built has resulted in the Senate Ethics Committee taking a look at the charges. But the Ethics Committee is composed equally of Dems and Repubs by design so it's almost impossible to get a censure. Probably the most feckless committee in Congress.