The explanation is that the number of devs (and many related trades) is increasing rapidly, largely driven by hires of young neophytes rather than retraining of e.g. 50 year old lawyers. The neophytes are younger and have lower wages than the mean dev. This brings the average down faster than raises and aging bring the average up.
You can verify this if you look at data for particular career cohorts, either people or experience levels. In the 4 years I've been on HN the quotes I hear for "generic CS undergrad from Stanford" are up more than 25% (80k to 110k or so, starting salary offer). Companies that I work with complain bitterly that their competitors are bidding irresponsibly. The going wages for interns are substantially in excess of what I expected to earn in mid career when I graduate not even 10 years ago.
Also, anecdotally, of the couple hundred people whose professional statuses I'm routinely exposed to I don't think I've ever heard "Boo, got to take a pay cut" except for making the BigCo to startup or self-employment transition.