"Momentous" means "a moment of great importance." In large part due to Faulkner's poetic style, every moment in "Barn Burning" seems important. This helps us feel the fragile nature of Sarty's perilous existence. If he doesn't do something big, and fast, he could easily become a casualty in his father's class war. The momentous tone also puts us in the moment.
As we learn in paragraph 29, Sarty is still in that moment twenty years later. As such, the tone of "Barn Burning" is also memorial. Although "Barn Burning" isn't presented as a memory, the brief glimpse into the future, shows that a constant memorial to his father and his lost family is playing out in Sarty's mind. Likewise, that moment on the hill, with his back to his old life, and his face to the woods is a good case in point.
The narrator constantly describes Sarty's emotions and his sensory experiences. The story's highly emotional and sensory tone is established in the very first paragraph when the hunger induced by the smell of cheese and the sight of the cans of meat in the store/courtroom, combine with the "fear," "despair" and "grief" (1).
Notice also that Sarty if often described as being so emotionally panicked that he can't see, hear, or feel anything, not even physical pain. By calling on the readers' senses, and contrasting dulled sensory experiences with acute one atone of emotional and sensory intensity helps move the story to its final momentous moment. In the final paragraph, Sarty's senses are alive with the fresh spring air, the sound of birds, and the "dark woods" he seems about to enter (109).