Possible reasons for this anomaly:
- Loss of faith in USD Teether - Thanks to Bitfinex
- Profit bookiNg in bitcoin, investors buying stable asset class (Sbd) making it unstable in the process
- Cannot buy traditional assets gold or USD for taxation concerns
I'd say the tether issue might be a possibility, however that's also been a concern for quite a while now. Also, if that were the case (for any of the points you described), it would also seem there would be a run on bitshares' BitUSD asset as well, but so far that doesn't seem to be the case...
(bitshares BTS/BitUSD chart)