At first glance, we may be tempted to conclude that moderately brisk weakness in stocks at the opening bell is the motivation for bonds to hit their best levels of the day shortly thereafter.  While there may have been some small measure of extra bond buying demand that came courtesy of stock selling, it's actually easier to make a case for bonds leading the charge and stocks finally giving in.

This can be seen both in the shorter term where overnight bond strength preceded the drop in stocks:

as well as a slightly longer term chart where stocks look like they may be giving up on their attempt to break the ceiling at the 2930-2940 range in S&P futures:

The unfortunate byproduct of this otherwise pleasant bond rally is that it reintroduces MBS underperformance after a few days of healing (i.e. Fannie 3.0 prices are up just over an eighth of a point while 10yr note prices are nearly up half a point).  Al of that is to be expected though.