One fairly facile way of making a case moot -- or potentially moot, anyway (hence justifying a remand to the district court) -- is to simply wait it out. That way you won't have to write an opinion.
So Anna Smith challenges the metadata program of the USA PATRIOT Act in 2013, and an appeal of the district court's decision is filed in 2014. The oral argument of that appeal transpires later in late 2014. So in normal circumstances, you'd expect an opinion sometime in 2015.
But the panel instead waits. Because the PATRIOT Act was set to expire in June of 2015. It got replaced by the USA FREEDOM Act, which was slightly different. Plus there were some intervening decisions of the FISC.
So, come 2016, the Ninth Circuit is ready to dispose of the case. So today, it dismisses part of the appeal and moot and remands other portions back to the district court for an initial determination of whether the remaining portions are moot as well.