Every year season spirits have to complete a ritual for one season to end and the next to start, the Spirit of Spring has used this as an excuse to express his hatred for Winter and to torture. And Bunny has just learnt that Jack is not a mere snow sprite, he is in fact the Spirit of Winter and wants to stop the ritual from happening.
What if when Jack was brought back by the Man in the Moon he still had all his human memories? He knew who he was and that he had a loving family... they just couldn't see him anymore