Know? Some, not everything. When any device is manufactured there are likely oils, not quite used chemical reactants, etc. Those items aren’t as stable as the device itself and they slowly evaporate (or react with the air, etc.) Anyway since there’s a finite amount of these “extra” things we get the usual asymptotically ramping down of the “pollutants”. If any of those items have any significant conductance they can be changing the value of a resistor, changing the parasitic resistance of a cap or inductor… Also these processes typically happen faster with higher temperatures.
Some other things like dielectric absorption are essentially mechanical changes (rather than chemical) and also proceed at a rate that proportional to things like the electric field strength.
There are other things like a buildup of magnetic fields in magnetic materials that can plausibly cause some of the “burn-in” effects.
In the DS in particular I kept the temperature down almost everywhere so any of the heat related changes take a lot longer than most audio devices. (Especially the output transformers which are far from heat and relatively huge compared to anything else audio related.)
For some components the data sheets give charts for how certain parameters of the device stabilize over time - even things as simple as resistors, but much more commonly in things like electrolytic caps…