Although these satellite's batteries have long been expired, because of some sort of chemical reaction due to thousands of failed recharge cycles the batteries begin to conduct over time and allow the satellite to be powered directly from the solar panels thus activating the transmitter.
Most are not transmitting any usable telemetry or weather images but they still use the original frequencies to sent out a unmodulated carrier, which interferes with the current operational Satellite's in the 136/138 MHz band. Normally when a satellite goes out of service and runs almost out of fuel the last reserve is used to shift it in a graveyard orbit which resides above the normal operation orbit, it will be switched off and left on their own.
But this will not always goes as planned, sometimes due solar radiation or other technical failures it will not respond again to the ground station commands, which leaves the Satellite in its current orbit. Some have a timer onboard which will be activated at end of life scheduled to end/shortcut the power feed to the transmitter so it will not broadcast again, also this can fail.
Archived sounds of the death throes here.
That first sentence bothers me, because it seems to imply that the batteries and the rest of the satellite's circuitry are connected in series with the solar cells. In reality, I'd expect them to be in parallel (probably with some extra complexity for voltage regulation, overcharge protection, etc.). In which case, it would be normal for the satellite to work when the batteries are dead but the solar cells are illuminated (just like a cell phone with a dead battery can often still work when the charger's plugged in).
see also