Because he has just announced that he will be forcing a vote on the Senate floor on the Paul Ryan budget, which dismantles Medicare:

Senate Majority Leader Harry Reid said it’s definite: He will scheduled a vote on House Republicans’ fiscal 2012 budget plan, which passed the House on a party-line vote earlier this month and includes the proposal to make Medicare a voucher-style health-care program for seniors.

The move could create an awkward situation for some Senate Republicans, who would anger conservative constituents by voting no, but could alienate independents by voting yes. Sen. Susan Collins, a centrist Republican from Maine, said last week she’ll oppose the budget.

Senate Republicans “seem to be in love with the [Rep. Paul] Ryan budget, they’re going to get an opportunity to vote on the Ryan budget,” Mr. Reid said. Asked if he thought the Senate would defeat the House GOP budgetary plan, Mr. Reid said “I hope so.”

An “awkward situation”, yes, I guess it is.

BTW, the Senate rules does not allow a filibuster of the budget.