There was a lot of volume at the close that probably had to be settled. This is not uncommon for the closing price to change within a few seconds of close when the volume is so high all at once. Everyone was trying to position trades based on derivative positions right at close. In this case it was most likely call positions where traders took on a short position for protection, that later covered in AH, thus pushing the stock up significantly in AH. It will be interesting to see how this works itself tomorrow when real volume comes in.