In the past few moments, 10yr yields ticked up above the domestic session highs and the yield curve began to steepen back to the morning's highs.  Both of these developments can be found in the early stages of broader selling pressure in bond markets.  Both of these things also typically happen right before MBS prices get on the bandwagon (of pain).

In today's case, opportunistic (or evil, or simply fat-fingered) MBS traders thought they'd take advantage of the potential panic by making it look like such a sell-off was taking shape.  Buyers entered bids (prices at which they'd be willing to buy MBS) that were much lower than the recent executed trades.  Bids can be seen in the white line below while ASKING prices (what sellers are willing to sell for) are in orange.

As you can see, sellers have had perfectly steady hands, and haven't been even remotely willing to swoop down and validate the silly bidding behavior seen in the white line (incidentally, the blue line is the spread between the two, and as it rises back to previous levels, it signifies the end of this weird little song and dance). 

As always, check in with THIS PRIMER for more on this phenomenon.  The quickest/easiest acid test is to ask yourself if 10yr yields are conveying the same level of panic seen in MBS.  If they're not, you're likely looking at one of these instances of bid/ask discrepancy.  Just another reason we follow 10yr yields so much, even though though "MBS" is right in our name.