the Dec contract has been out of line with U08 and M09 for a while now, this isn't a recent phenomena; look at the Z8U9 spread, it now trading @ -2.5, right out of joint with the rest of the curve.
As for why, it might just be a seasonal effect, compunded by the added uncertainty in these markets. Also remember that the U8 contract has now gone quarter tick, which may well change the dynamics.