I know that for moisture ,- and that is the problem sometimes, but I'm talking about the idiots that I watch in restaurants that fill salt shakers (and even worse pepper shakers) so that it is mounding over the lip and then screw down the top so sometimes it actually compresses the product, leaving little if any air space between top of product and the holes - to the point if you take the top off, even holding it level, it spills out. Some grains won't fit through holes and when these are pushed tight against the inside of the top - without room to "shake" it gets mechanically clogged.

First world problem - but easy solution - leave an air space.