It mainly caused when the gun and product doesn’t have much space and particles emplacement leads to back ionization.
Often, this issue happens in corona spraying guns, because the charging part is placed at tip of the gun, when the powder passes through the tip (on close coating that is less than one feet) the ions are negatively charged and grounded, some particles will jump over and form back ionization.
In Tribo charging gun back ionization will not happen, because the powder particles got charged inside the gun (as internal parts of the gun are made up of Teflon). But on the other side, powder output will be less when comparing to corona charging gun.
When the gun and the part are very close, the high voltage powder particles collide with air molecules that split them into negative components (electrons/ions) and positive component (proton). The ions will flow to the grounded part and protons are attracted on negative electrode and make them to stick to the surface.