The 5yr Treasury auction was strong, but not strong enough to account for what we're seeing in terms of trading momentum.  More to the point, simply glance at a 2-day chart of Treasuries or MBS to see that today's move has been a linear and decisive move in one direction.  

There are two factors in play, in addition to the strong auction (which, again, isn't the market mover you're looking for).

The first consideration is that Thursday is a half-day, which leaves tomorrow as the only proper "month-end" trading day (click that link to learn more about why that matters).  That means there can be a certain amount of bonds that will simply be bought (or sold, in the case of shorter maturities) over the first 3 days of this week as traders get in position for month-end.

The 2nd consideration is the fuel on the fire.  It has to do with the immense "short" base in bond markets.  In other words, just as we've talked about the fact that bonds would have a hard time making a meaningful, sustained break below 2.80% in 10yr yields, traders have increasingly made bets on yields moving higher.  In other words, they're "short" bonds.  An overabundance of short positions is susceptible to any forced improvement in bonds, whether it's driven by a decent auction, month-end buying, mild stock losses, etc. 

A trader with a short position covers their bet by buying bonds, which, in turn raises prices and lowers yields, thus potentially acting as a trigger for the next short in line.  As those dominoes fall and as that snowball rolls, the rally picks up momentum.  That's the gist of today's strong move, although it's worth remembering that it only prolongs the same old consolidation pattern we've been watching, even though it's pushing the lower boundaries.  Yields would need to close tomorrow in the same territory or better to confirm a new move is underway.  Even then, we run the risk of April's "new month" momentum taking things in the other direction.