Apparently it went into a tumble at the end of the rocket burn but recovered as it began reentry. I was afraid this would happen, I'm glad they recovered from it. I think SS1's vertical CG isn't on the centerline, probably because they added extra mass to meet the X-Prize requirements without balancing it. So they had a steadily increasing pitch torque as the propellant burned off and finally couldn't hold it steady.
It's past the danger now. Should be an easy fix. They'll have to balance the payload before launch (which will be a pain to analyze) or have some active ballast (ie, a weight on a ropes and pulleys contraption) so they can adjust the CG in flight.
UPDATE: Okay, I'm now seeing a report that SS1 was rolling. If it was tumbling on the roll axis instead of the pitch axis CG would have nothing to do with it.
FURTHER UPDATE: firedrake_mor was watching the video and tells me the nose kept pointing up, so CG problems had nothing to do with the roll problem.