The Greek government's poor financial standing reduced demand for its bonds, as shown in the graph below:

Interest in Greek bonds will fall from BD to BD' as financial experts lose faith in the Greek government's ability to repay its obligations due to its high spending emergency. The price of Greek bonds then falls from PG to PG'.
Meanwhile, the fall in demand for Greek bonds increases demand for US bonds, as shown below:

Demand for US bonds increased from BDto BD' as investors sought alternative investment options as Greek bonds became riskier. As a result, the price of US bonds rises from PUS to PUS'.
The risk premium between US Treasury debt and comparable maturity Greek debt exists because Greek bonds are more risky to invest in, so investors demand a higher rate of interest to compensate for the additional risk they are taking on. Bond prices and interest rates are inversely connected, so keep that in mind. The fall in the price of Greek bonds indicates an increase in the country's interest rate.