Like just about any other asset, Treasurys are affected by supply and demand. Investors often turn to these government-backed securities during times of economic uncertainty. Higher demand pushes prices higher, but it also means Treasury yields are lower because bond prices and yields are inversely related.
Imagine a Treasury with a face value of $1,000 and a yield of 4%, meaning it pays $40 in annual interest. Due to increasing demand, the price rises to $1,050. Since the Treasury still pays $40 per year, the yield has decreased to 3.8%. On the other hand, if the price fell to $950, the yield would go up to 4.2%.