Has the plated surface of the formerly lead engine's tires changed noticeably? Maybe the coating has worn sufficiently that the traction has suffered just enough to cause the locomotive to spin.
Also, I don't buy that the tracks haven't changed. At the very least they have worn about as much as the tires under the locomotives. But also, over time, EVERYTHING is fluid. Everything 'runs', or sags, or deforms, over time. Even daily or seasonal changes in ambient temps and humidity will affect our wooden supporting structures on our layouts. It's why I run a dehumidifier in my train room year-round. It may go several weeks without cycling into the drying process, but it's always on, ready to cycle and to commence the process. Some times I go into the train room to find the red light illuminated and the machine silent, indicating I need to empty the now-full reservoir. I had no indication that the humidity had changed and that the machine had been working for some time.
One other possibility, but I'm at a loss to explain exactly how the physics would work: maybe the drive mechanism of that former lead locomotive has finally broken in such that the drive is capable of delivering more tractive effort with the same amount of voltage, and drawing the same amperage. It has gotten to the point where it wants to spin where it couldn't before, and now you're seeing a 'freed' mechanism that wants to go faster, or to pull more, than you had it set for when it was matched with the consist. If you could couple each locomotive separately to a heavy consist and measure its amperage draw at a defined voltage, say 8 volts, and repeated the test with the other locomotive pulling the same consist in the very same location on your layout, also 8 volts, you might find a difference in either tractive effort or amperage. Trouble is, you'd have to be able to compare those figures with a previous test to really see if my suspicion is right.