Yield curve flattening- abstract question
If an investor has a fixed amount to invest for a given time horizon, then a longer-term bond locks in the current long-term bond interest rate, but a short-term bond only provides this rate for a short time. After that, the short-term interest rate may be more or less favourable. Apparently the investors expect that the short-term rate will become less favourable for them after the current short-term bonds mature. 

Investment is mostly about expectations, which may or may not be correct. Where the expectations come from is a more complicated question.