Mortgage rates began the day in roughly unchanged territory. Some lenders were microscopically stronger or weaker compared to yesterday, but not enough to impact the average mortgage borrower. For the first few hours of the day, it looked as if rates would stay unchanged or possibly move slightly higher. That all changed when stocks began losing ground.
It's always worth remembering (and this will be especially true when the next time it's proven) that there's no magic rule that says stock prices and interest rates must move in the same direction. It is true that there are frequent examples of such correlation, but there are plenty of other examples where the correlation complete breaks down. All that to say that stock losses helped rates today, but will not always necessarily help rates in the future.
Bond markets (which underlie mortgage rates) improved by enough this afternoon for most lenders to release "positive reprices" (where a lender recalls the morning's mortgage rate offerings and replaces them with less costly offerings). These reprices were modest by most standards, but they could be enough to make a meaningful difference for a prospective borrower who avoided locking last week in the hopes of a recovery this week.