French and his team have a hypothesis that might shed some light on the situation, and it's tied to the number of Saturn "moonlets" found near the F ring.
French’s team suggest that these tiny satellites, which are less than 5 kilometers wide, create bright clumps when they plow into the densest parts of the F ring. Consequently, the drop in bright clumps may indicate a big decrease in the number of moonlets; this would explain the change of look of Saturn’s iconic rings.
But why would the moonlets be more common in the Voyager era than in the time of Cassini? Researchers believe that it may be due to periodic alignments of Saturn's 135 kilometer long moon Prometheus and the F ring.
Both the F ring and Prometheus lie near Saturn's Roche limit — the orbital distance within which the gas giant's powerful gravity rips small objects apart, as understood by the researchers.
The research team consider that this periodic alignment may lead to a generation of lots of moonlets, which would, in turn, produce many of the mysterious F-ring bright clumps. These clumps would fade over time, the researchers explained, because repeated collisions with ring material would eventually destroy the moonlets.
Wall, Mike. "Saturn's Weird Fast-Changing Ring Baffles Scientists."
Space.com. N.p., Sept. 2014. Web. 15 Sept. 2014.