Rising long-term bond yields are evidence that

government borrowing is causing funds to dry up on that segment of the yield curve.
investors fear that the government will default on its debt or inflate it away.
investors are diversifying their financial asset portfolios as their confidence grows.

Answer: investors are diversifying their financial asset portfolios as their confidence grows.

Please read Fiscal sustainability 101 - Part 3 for further material or post a comment if you want further explanation.