Up until now, Greece has remained afloat due to the belief that the rest of Europe would bail them out. That belief took a body blow yesterday when the ECB said it wouldn't change the rules for Greece.
Because of this, the Euro tumbled today, while yields soared for Greek debt.

However, the real shocker for the market was the revalation that Greece was "looking at everything" in order to meet its borrowing needs. This indicated that it wouldn't be able to afford the higher interest rates in the not too distant future.