It's the return current for the DC trying to find its way into the return path for the AC. (Or vice versa, can never remember). Causes electrolytic corrosion of the rebar in the foundations and fixing bolts for the OLE masts. The old masts were pulling out quite easily. I spent a Sunday at South Acton a decade ago pouring concrete for new foundations, the bolts had special insulation.
It's the former IIRC.
What is happening is that eventually the centre point of the DC "Rail" and I'm on about rectifier rail here is at the same potential as the AC Ground, or neutral point. The return current from the DC system leaks and leaches into everything around it, finding the easiest return route to the rectifier, and any other route.
If you think back to GCSE or O Level Physics, with parallel resistors, if you think of a 47R and 4R7 in parallel, most of the current will flow through the 4R7 resistor, but some current will flow through the 47R one.
One can model the 47R as the ground and services around the desired current return path (the running rails) as they are not insulated from ground, current will also flow through the ground.
When one considers that a single train will realistically have 3MW of traction motors per unit, or up to 9 or 10MW for a 12 car train, using the most basic forms of power calc you can see a current of around 13kA for the latter. A single unit being around 4kA peak current draw. Even if the resistance of the ground is 1000 times that of the return path via the rail (it isn't, it's a lot less in reality, even less in areas with AC electrification) there is still a meaningful current being passed through the ground.
Next up we bring into account the fact that we have different materials in contact, (as you always will). The current flowing in a certain direction, all the time, passes electrons from one material to another constantly, and through chemistry I can't fully explain, passes corrosion on from one place to another, known as electrolytic corrosion if I remember correctly.
Now when we have an AC and DC system that are not intentionally connected to each other (as they shouldn't be in an ideal world), the DC leakage will flow in and out of the AC return paths and earthing routes, as the OHLE supports and earthing connection through them is of a significantly lower impedance than the ground, this attracts more of this corrosion to the boundary points where this stray current is able to flow.
LU 4th Rail systems do not suffer this issue (most of the time) as the return route to the rectifier is through the centre (4th) rail, the traction current is essentially (although it actually isn't) isolated from earth and the running rails, both on the track and the trains.
Bear in mind it is nearly half midnight so please forgive any mistakes as this isn't what I do on a day to day basis and a lot of it is remembered from a couple of reports I put together a few years back.
