As you know the fluorescent bulb contains a gas that produces invisible ultraviolet light (UV) when the gas is excited by electricity. The UV light hits the white coating inside the fluorescent bulb and the coating changes it into light you can see. Because fluorescent bulbs don’t use heat to create light, they are far more energy-efficient than regular incandescent bulbs, therefore the bulb could still be burning residual gas inside the bulb with a little stray current, hence the flickering. The stray current can be created due to tuning of inductance/resistance of circuit (ballast and bulb electrical discharge path) with variable stray capacitance. Stray capacitance amount is very complex and varied by different switching in circuit. 