Most all paper manufactured in the past one hundred years or more contains acids. If left untreated, these acids will slowly decompose the paper itself. The use of acids in the manufacture of paper did not become popular until the early 20th century. Older newspapers of the 19th century were printed on paper that had no acids so they tend to last much longer.
Newspaper clippings or any other documents not printed on acid-free paper will eventually disintegrate. Today's newspapers usually contain more acids than other paper so newspapers are often the first to disintegrate. Luckily, modern science has created methods of slowing down or even stopping the decay of such paper.