This is actually quite surprising...
I thought he was killing Keystone by dragging out the “NO”, and now what was implicit becomes explicit.
He must have hit the point where he figured a later rejection would be harmful to the Democrats in next November and would allow time for the Canadians to wait it out (imagine a January 19, 2017 rejection which could be reversed in two days). Now, the pipeline to the Pacific for the Chinese will be started immediately and won't be stopped even if this is reversed in 2017.