Theories for last minute switch by Obama on UN Arms Trade Treaty?

Assuming that this is true, why the last minute switch?  Politics with the election coming up?

The UN had spent the entire month of July hammering out a deal, and Friday was the deadline for an agreement on a treaty that has met with the staunch opposition of the National Rifle Association and bipartisan concerns in the Senate.   

“This was stunning cowardice by the Obama administration, which at the last minute did an about-face and scuttled progress toward a global arms treaty, just as it reached the finish line,” said Suzanne Nossel, the executive director of Amnesty International USA. “It’s a staggering abdication of leadership by the world’s largest exporter of conventional weapons to pull the plug on the talks just as they were nearing an historic breakthrough that would have required all nations to deny arms export licenses where there was an overriding risk that the weapons would be used to facilitate serious crimes against humanity.” . . .