There is no prepping
this bad thing happened.
As I quoted, the purpose of prep is to give the GM things to say.
Part of the apparatus of prep, as both
@thefutilist and I indicated in our posts, can be a countdown clock. The countdown clock includes bad things happening. The GM says those things when (i) they make a move, and (ii) because they always say what prep demands, therefore (iii) they look to their clock and say something that it tells them to say. If it's time to make a soft move, then - as I posted upthread in reply to you - maybe they tell the player that their PC sees Krumptown on fire. If it's time to make a hard move, then - as I also posted upthread in reply to you - maybe they tell the player that they see Krumptown burned to the ground.
I'm not really getting the source of confusion - this is all set out in the rulebook.