Possibly the American Civil Rights Movement.
Living as he did in Tennessee, in the American South, Foote would have seen a lot of changes during the decades surrounding his writing of The Civil War: A Narrative. The fact that he was writing about the US Civil War, as a man from the South, would only have complicated this further for him. From the article "Shelby Foote's War Story: How a Memphis novelist’s history of the Civil War made history itself" by Jon Meacham (links added by me):
Foote undertook his narrative of what he would later call “the crossroads of our being” in the years in which the civil rights movement forced the South to confront the war’s worst legacy: segregation. He began work in the months after the Brown v. Board of Education decision came down, published his second volume in the year of the March on Washington, and completed the trilogy as the struggles over affirmative action were taking shape.
Foote also wrote himself, in notes on the second volume:
I am obligated to the governors of my native state and the adjoining states of Arkansas and Alabama for helping to lessen my sectional bias by reproducing, in their actions during several of the years that went into the writing of this volume, much that was least admirable in the position my forebears occupied when they stood up to Lincoln. [...] I suppose, or in any case fervently hope, it is true that history never repeats itself, but I know from watching these three gentlemen [Ross Barnett, Orval Faubus, and George Wallace] that it can be terrifying in its approximations.
It's understandable how current events in the world around him were so distracting to him, when they fitted so closely with the very events he was writing about in his historical narrative.