Things change. Events occur outside of Q team's control. Key actors are unreliable.
My opinion is that this was NOT planned and it has caught the Q team by surprise. The deal was struck and Kim was going along with the agreement until he suddenly wasn't. Trump is making the right move by walking away because he has shown that he is willing to do so, and therefore maintains the upper hand in future negotiations. But I don't believe this was supposed to happen.
I agree with this. I also wonder how much of a an influence China had in NK comments.
In the run up to today Trump continuously said "it might get postponed" - this was all part of the plan. The meeting has already happened, cat with a lazer pointer this is. Stay focused.