Mortgage rates generally rise when investors have a flight to safety mentality.
A flight to safety happens when investors are nervous about owning risky assets like stocks, but do not want to miss out on earning a return on their funds, so they move their money into risk-free U.S Treasury debt to provide a safe-haven AND an return on investment.

To remind readers, as Treasury yields fall, prices of mortgage-backed securities move higher, which allows lenders to offer lower mortgage rates. As Treasury yields rise, mortgage-backed security prices are led lower, which forces lenders to push mortgage rates higher.

Of course there are many other factors involved in mortgage rates, but this is a quick and simple formula that mortgage shoppers can use to help them decide whether or not to lock in a rate. Are treasury yields moving up or down? Mortgage rates will likely move in the same direction.