The large momentary electric current makes the dim purple plasma explode with light and sound. This is because the growing motion of lightning and sparks is actually the growth of plasma filaments. It is not a movement of light. In very large Tesla Coil systems, the giant sparks can lengthen VERY slowly, a human can sometimes outrun them. This is hard to explain without going into detail about electric fields and atoms.
Cracks tend to start at defects, and a similar thing is true with lightning and sparks.
An invisible field of electric force, if applied to air, can cause plasma filaments to burst into existence anywhere in the part of the air where the field exists. When lightning is advancing towards the ground, there is a strong electric field all through the air around the plasma branch and in the space above the surface of the earth. This strong field can trigger new plasma filaments to grow anywhere. Of course its main effect is to make the main lightning filament lengthen and grow downwards.