Mortgage rates fell moderately today, largely in response to the Federal Reserve's policy announcement. The Fed is in charge of seeing a key short-term rate that impacts the entire financial market by varying degrees. The Fed does NOT set fixed mortgage rates, but in general, the friendlier the Fed with its monetary policy, the better it is for the entire spectrum of rates.
The Fed was quite simply friendlier than expected today. Investors were already planning on some sort of adjustment in the verbiage promising ongoing rate hikes and decreases in the amount of bonds purchased directly by the Fed. We got that, and more. For all intents and purposes, today's announcement and press conference could be interpreted as the Fed saying it's done hiking rates until further notice and would only resume hiking if economic data at home and abroad justifies it.
Bonds and stocks both improved immediately. When bonds improve enough, mortgage lenders often lower rates in the middle of the day. Today's gains were just enough for that to happen for a majority of lenders. The improvements bring rates back to levels seen 3 weeks ago, although much of that has to do with yesterday's levels providing an already-fairly-low baseline.