Really hoping for a big plot episode for Zekrom here, also hoping it's the same one Ash saw at the start of BW.
This could be a few things really:
-Leaving Unova properly now, Ash thinks back to his first day on the region which causes him to think of Zekrom.
-Ash see's Zekrom flying through the sky + OR it shocks him and Pikachu once again.
-Some other plot causing Zekrom to appear.