A sharp recent drop in the Fed’s holdings of U.S. Treasuries for foreign central banks probably reflects the effort by many developing economies to stem rapid declines in their currencies, not some frightening move by the likes of China out of U.S. bonds. That’s the argument put forth by Marc Chandler at Brown Brothers Harriman, who notes the pullback of recent weeks appears to have been the most dramatic since the Asian financial crisis of the late 1990s.

His reasoning makes sense: a September spike in the U.S. dollar was accompanied by steep plunges in the exchange rates of many emerging economies. Still, Chandler remains puzzled as to why the selling accelerated to a hefty $21 billion even as the dollar reversed course in the last week:

This is the seventh consecutive weekly decline and over this period, custody holdings have fallen an average of about $12-$12.5 billion a week, making this past week quite large relative to trend. It likely reflects foreign central banks’ selling of Treasuries to intervene to support their currencies rather than a dumping of Treasuries to diversify reserves or as a protest to such low interest rates.

Yet the difficulty with this hypothesis is that during the week through Wednesday, most emerging market currencies have generally risen against the dollar. This generalization holds true for East Asia which is suspected to use the Fed’s custodial services. For some of the run the dollar was appreciating in general, so private sector dollar buying offset the official selling, but now — over past week — it would seem like the central banks and the private sector have been on the same side selling dollars.