One thing I don't think I've see mentioned is that as Ansel aged, his eyesight started to fail. As a result, he may have been striving for more contrast in his prints, which led to heavier printing...

Carlos Hererra (who used to run the Workshop program in Yosemite), through conversations with people who used to work with Ansel, was told that the workshop assistants used to do the final toning of the prints, since Ansel would take them much too far - he just couldn't see as well to judge when the prints where 'done'.

So, in addition to a printers interpretation changing over time, physical changes may also play a role...