1. Investors start to fully appreciate the fact that the U.S. gov’t is already technically bankrupt. Liabilities>Assets on a massive scale when you add in Medicare, S.S., existing debt, bailout, etc. Increasing taxes is not a vehicle to raise the money-we’ll hit the wrong side of the laffer curve. Gov’t will ultimately decide to screw the debtholders by printing money (already in process???).
2. Economy improves and investors who are currently holding the safest assets they can find (treasuries) become less risk averse. Demand decreases and yield rises.
3. China/Japan/Middle East gov’ts lose their appetite for treasuries do to the need to stimulate their own ecnomies and an increasing appreciation (rationality?) for #1 above. Demand decreases, prices decrease, yields rise.
Scenarios I can come up with where the bubble doesn’t burst only hold up for a year or two (continued economic weakness, government buys treasuries, etc.) After that, one of the forces in #1-#3 takes over.