Depending on how you want to examine the story, the climax comes after Jonas has learnt the "truth" about his society and the way that the tightly controlled nature and rules and regulations are reinforced by the euphemistically named punishment of "release," which is actually death. Having learnt this, and then having decided to work hard to try and change his society, there is a sudden break in the plot at the beginning of Chapter Twenty-One, indicated by the following paragraph:
But that evening everything changed. All of it--all the things they had thought through so meticulously--fell apart.
Jonas was "forced to flee," in the words of the novel, because of the imminent "release" of Gabriel. The love that Jonas has for Gabriel and the way that he has been sharing memories with Gabriel has created a bond between them that Jonas cannot simply ignore. This is what leads to his escape and the kidnap of Gabriel from the community and their subsequent flight into unknown territory and the highly ambiguous ending as they sled downwards into an uncertain and unknowable future.