Almarg, good man, how about it?I haven't the foggiest idea :)
Some people, most likely non-EE's, would claim that during the period of time the power cords were unused they lost their breakin, whatever that may mean technically, and that the change you noted during the first days of re-use reflected re-breakin. Which with your particular system happened to work in the wrong direction sonically.
A rationale that is commonly offered for the breakin phenomenon, again usually by non-EE's, is something called dielectric absorption. As you'll see in the linked writeup, that is a phenomenon that occurs with capacitors, and since any cable or power cord has some small amount of capacitance, some audiophiles have concluded that it is applicable to cables and power cords as well.
However, I have never seen any explanation that plausibly establishes in a QUANTITATIVE manner how dielectric absorption may be significant in the context of audio system cables or power cords. Nor for that matter have I ever seen what I consider to be a plausible quantitative rationale for any other hypothesized cable or power cord breakin explanation.
Best regards,
-- Al