First, we know that he gave 23 years of taxes to John McCain, back in 2008. McCain has said that taxes weren't the issue, which isn't worth much, but the point is Romney really didn't object to revealing his taxes then.
Next we see that Romney is willing to release the last two years, and has already admitted he paid about 13% throughout the last decade. He's doubled down on the claim that he never paid 0%, which isn't worth much, but the point is that if all he had done was escape with a low tax rate, he's basically already admitted it.
Finally, we see the Obama campaign promising to stop asking after the last five.
All of this points to exactly one year: 2009. And what happened in 2009, that hadn't happened in 2008 and indeed didn't look like it would ever happen then?
The Swiss cracked. They agreed to open their books to the IRS. And tax cheats rushed to sign up for the amnesty program, trading a hefty penalty for immunity to prosecution.
My prediction is that Romney took the amnesty. If this comes out before the election, it will kill him; if it comes out after the election, it's an impeachable offense. And the idea that it won't ever come out is left over from the Nixon era. Which describes Romney, actually.