I smiling at the moment ... I'm now seeing a few threads on my former haunt at audiophilestyle.com along the lines of "Why does my system start sounding a bit off after some time powered up?" ... oh dear, 😉.
I spent years on this, trying to get answers. My first good system had this really, really bad ... I never fixed it at the time, and got so discouraged I completely lost interest in high quality playback. Finally, climbed back on the saddle, and tried again ... more experiences, experiments, answers ... much better place of understanding now.
Why does it happen? Unfortunately, a number of things ... one of the easiest to understand is that the old style potentiometers used for volume control, etc, are a bit of a disaster for SQ; the way they're constructed is using poor quality surface to surface mechanical contacts, and the point where the surfaces meet deteriorate in electrical integrity over a relatively short period of time; the weak solution is to adjust the control, to refresh the contact areas - the better the setup in which they're used, the easier it is to hear the effect.
Another, more pernicious villain is static building up a charge, somewhere, which is disturbing the correct operating voltages for circuitry - they slowly move from their optimum value, to one where audible distortion becomes obvious.This is why going through a powering down, resting, and powering up again cycle always works - albeit, a very annoying 'fix' to have to rely on.
What to do about it? No easy, step by step answer - it can be a long, hard slog tracking down the cause(s). The best solution is to invest in gear which is immune to the problem; otherwise, start doing some 'debugging' - there will be an answer, eventually ... main thing is to have the patience to keep trying ideas, so that a good solution is the end point of one's efforts ...
So, why has it taken so long for more people to start asking,"What, is going on ... ?!!" ... I don't know, perhaps part of it is that only recently have systems in general advanced enough in SQ, for this to be more obvious as an anomaly, to more people.