We've definitely seen Treasuries improve relative to MBS more often than not in the past week or two.  The most obvious driving force behind that phenomenon was fairly simple.  It had to do with MBS outperforming even more noticeably in the 6 weeks before that (a feat which was incidentally attributable to MBS severely underperforming during the initial coronavirus volatility.  In other words, MBS did so much better vs Treasuries that it made sense for the relationship to equalize a bit.

Another major consideration is and has been the Treasury supply...