Treasuries tanked today and there were none of the sort of obvious catalysts you'd hope to see.  I checked my math on that with people smarter than me and the conclusion was the same.  Granted, we can all point to things that probably aren't helping: some anecdotal trade deal traction over the weekend, stronger EU econ data overnight, new month tradeflows, and supply-related concessions.  But if Treasury yields were lower today, there would be no reason to mention any of those things.

Perhaps the only thing truly worth mentioning (apart from the market movement itself) is the fact that MBS were almost totally immune from the Treasury trouble.  This adds to the case that "supply" (Treasury auctions and corporate issuance) is a factor in the sell-off.  But MBS have been so weak vs Treasuries in the bigger-picture that they don't need any major reason to outperform when bonds are selling off but remaining inside last week's range.

Actually, it's also worth mentioning that the week's most important economic report is coming out tomorrow morning at 10am ET.  ISM Non-Manufacturing will have a chance to add to today's weakness if it beats the 53.5 forecast, and perhaps even if it only improves on the 52.6 previous reading.  If it misses the mark, it will be interesting to see how much bonds are willing to react.  If "not much," then we'll really know that the market is tripped up by this week's supply of new issuance.