What if the Night King raises a bunch of dead Starks, which turns most of the people in the crypts into wights (some of the named ones might escape who maybe notify the rest of what's happening), leading to Dany making a calculated decision to nuke Winterfell with her dragons to keep the wights from overrunning folks killing some other people in the process (maybe Arya or Sansa?), which is what pushes Jon to kill her?
It would be an extreme enough act to get Jon to turn on Dany while also being something I could totally see Martin doing.