Investors can't change a bond's interest payment. So if they think inflation is looming, they push the bond's price down, which pushes up its yield. Inflation is often the byproduct of an overheated economy. So when traders think inflation will slow, they push bond prices up and yields down. Short-term bonds aren't as volatile as long term.
Investors demand higher yields from issuers with low credit ratings.
The U.S. Treasury has the best credit. Junk bond issuers have the worst. Too many new bonds on the market pushes prices down and yields up, and vice versa.