Salt and Sugar are both Hygroscopic, meaning they can pull moisture from the air. If put up with a high moisture content they'll clump up. use an O2 absorber and the partial vacuum they create just put force on all the grains so they clump up even more. Even to the point of becoming a solid brick.
They both can be broken up again.(ice pick, hammer, chisel) More of a problem if done in mason jars as your more likely to break the glass getting it out.
I always put all my dry food to be packed in a room that has a dehumidifier and run it at max for several days just to make sure everything is really dry. The low moisture air will pull the moisture out of sugar and salt. Does a good job one everything else too.