What is the physical explanation of electric breakdown?
In a storng E field, the free electrons in the air or in the gas cloud begin to accelerate until they hit the molecule. If the kinetic energy of accelerated electron is such to expel the other electron in the molecule then you have now two electrons and one ion, positively charged.
If this gas cloud is relatively low density, with lot of free space between molecules, the electrons can gain higher speeds prior hiting the molecule thus more probability to ionize, you get a chain reaction - breakdown. As the ions have the most mass, they are very steady, they can't accelerate or move as electrons, so the most flow consists of electrons which are very movable.