On page 163 of The Giver, why did things "fall apart"?

It is always problematic using page numbers as depending
on the edition of the book they may be different. In future, please reference particular
events by refering to specific chapters. From your question, I am guessing that you are
talking about the events of Chapter Twenty-One, in which Jonas is forced to swiftly
leave the community which has been his home all of his life due to the announcement made
by his father that Gabe is going to be released the very next day due to his lack of
progress and development. Let us remember that Jonas has just discovered the true nature
of being "released" thanks to the video that the Giver showed him of his father
releasing the twins before. In spite of his plans to slowly work on changing the
community from within, this forces the hand of Jonas and he is forced to run away from
the community with Gabriel in order to save his life.