"I have no idea," said Sen. Debbie Stabenow, D-Mich.
After all, the proposal to allow younger Americans to buy-in to Medicare had been crafted specifically as a compromise for Lieberman and other moderates who opposed a public option, reports CBS News correspondent Nancy Cordes.
Lieberman has championed the buy-in idea for nearly a decade and reiterated his support just three months ago.
"What I was proposing was that they have an option to buy in to Medicare early," he said during an interview.
But this week he suddenly threatened to block the health care bill if it contains a Medicare buy-in, throwing the entire process into disarray yet again.
"I'm very disappointed because I thought we had Sen. Lieberman's staff in the room, I thought during the negotiations he would go along with it I'm very disappointed that he didn't," said Sen. Sherrod Brown, D-Ohio.
It is especially mystifying because Lieberman has historically sided with liberals on domestic issues, even though he became an independent in 2006 after losing his Democratic primary.
"No one can understand exactly what Joe's up to with this," said Jim Shea with the Hartford Courant. "There's some people that think that he's grandstanding. There's some people that think he's cow towing to the insurance industry."
The industry does have a large presence in Connecticut and gives generously to his campaigns. But Lieberman is hardly the only moderate putting the brakes on the bill.
Democratic senators Mary Landrieu, Blanche Lincoln and Ben Nelson, who came from conservative states successfully blocked the public option, even though the other 55 Democrats support it.
Nelson is still holding out for stricter anti-abortion language.
It is all leading some liberals to complain their agenda is being hijacked by a few.
"Does it trouble you that you're going against the overwhelming majority of your caucus?" Cordes asked Lieberman.
"Well it's not fun," he said. "I said that to my colleagues at the White House, I haven't enjoyed it. But you got to do what you think is right."
So this small group of four or so senators has managed to impose their will on 530 other members of Congress because the changes they're demanding in the Senate bill will likely have to be made to the House bill, too. But Congressional experts say this is exactly how the process is designed to work - it's designed to require compromise.