Mortgage rates moved just a bit lower today. Relative to any other day in the past 2 weeks, it was unremarkable.  But because the range has been so narrow over that time, and because rates were already at the lower boundary of that range yesterday, it technically resulted in the lowest average rate since Fed Day on September 17th.

The underlying bond market was slightly weaker. This would typically result in mortgage rates moving higher. The catch is the timing of the weakness (and yesterday's strength). 

Specifically, bonds improved yesterday afternoon but not enough for the average lender to change its rates for the day. Today's bond market is weaker compared to yesterday afternoon's levels, but stronger than yesterday morning's levels (when a majority of mortgage lenders published rates). 

In other words, today's drop in rates had everything to do with yesterday afternoon's bond market gains.  All that needed to happen this morning was for bonds not to lose too much of that ground.