Mortgage rates are not tied directly to the 10-year Treasury, but the two often move in the same direction because both compete for long-term investment dollars. Mortgage-backed securities usually need to offer investors a higher return than Treasuries because they carry different risks, including prepayment risk.
That is why changes in the 10-year Treasury can be a useful clue about where mortgage rates may be heading, even though the relationship is not one-for-one.