There is little doubt that ZAB's hanging had political rather than legal reasons, which should have been revisited a long time ago. His untimely death was an irreparable national loss. But AAZ instead of properly investigating BB's assassination, took legal refuge in the presidency from the NRO onslaught.
And instead of following ZAB's footsteps of public-friendly policies, he distanced himself from the public's needs and kept sliding down in ratings related to their sentiments. To make the situation worse, he kept to his past practice of keeping corrupt buddies around him, thus isolating his political path from the one followed by ZAB. So, it looks like the reasons to reverse the verdict are the same - political rather than legal.