Capacitance on the wiring to the mains switch, which is by you a parallel pair of cables in a conduit. This few hundred pF to 2nF capacitor is enough to charge the capacitors in the lamp till it lights dimly. Some lamps will flash at around one flash every 2-4 seconds as they build up enough charge to start the SMPS inside the lamp.
Solutions are a bleed resistor of around 470k 1W across the lamp or an incandescent lamp in parallel, or separate the wiring into 2 separate conduits, or use 4 wire cable, with neutral and earth, to every switch to provide shielding. It is not common on old installations with steel conduit as this attenuates the phantom voltage a lot with the capacitance of the cables to the grounded conduit making an attenuator with a higher capacitance to the cable than between the cables.