It was most likely a plan but even that was iffy writing. It was heavily implied that the only reason the wights just stood there waiting was because the ice was breaking, so if the charged they would end up in the water. When The Hound throws that last rock. it bounces easily because the cold has made the ice strong again. So the wights go
and start charging slowly at them, without any sign of a dragon approaching. I do believe the NK was waiting for a Dragon, but he should've made the wights stop until Danny showed up, imo.