pardon this bastard explanation, but as my grandfather explained to me:
old crts generate a pretty substantial static field when fired up (place your arm near the screen and you can see your hairs lift) when powered off that energy is still there but the cathode ray is now off causing a surplus of (negatively) charged particles in a concentrated area and they wish to disperse into equilibrium and as they do there are small arcs of electricity and that is the crackling sound you hear.
i had the same trinitron tv back in the day, man i need to reacquire on for my lightgun and beat based games