I'll take a stab grov. Because the major holders of US debt see this as a perfect chance to offload some of their holdings before the dollar declines to much.
Because the US needs to borrow so much, with a lack of demand (which is evident with them purchasing their own) and an increasing supply, where do they stop? They will end up having to buy such large quantities that is not economically possible for their dollar not to devalue substantially.
