In this video of my bug zapper (Which is now at my father home), I discovered that the electrodes have a thermionic discharge, even before they reached their optimal temperature, during the preheating.
How this can be happen?

In this video of my bug zapper (Which is now at my father home), I discovered that the electrodes have a thermionic discharge, even before they reached their optimal temperature, during the preheating.
How this can be happen?

Maybe some Instant-Start combination. Can you take it apart? Possibly the starter is used for something else.

In this video of my bug zapper (Which is now at my father home), I discovered that the electrodes have a thermionic discharge, even before they reached their optimal temperature, during the preheating.
How this can be happen?

There is some thermionic discharge just before the starter strikes the lamp, that is pretty normal. The starter keeps closed for a predetermined amount of time, it does not sense in any way the real state of the electrodes. So if the ballast has a bit higher preheat current (many ballasts do slightly saturate when exposed to full mains voltage during preheat), the electrodes may actually reach the thermionic emission before the starter opens.

And if the question is about the purple glow just when the thing is powered ON (before the starter closes the first time), that is cold cathode glow discharge (and the purple is the glow of the sputtered electrodes). It could be either because the "upper" starter closes first and so connect the full mains across the bottom lamp (these lamps tend to even cold start on 230V mains when there is no starter), and/or because there is quite strong capacitive coupling (between the wires or from the grid to the lamp gas fill) from the HV grid supply (it may aid the cold ignition).

I means that the thermionic discharge appears, before the electrodes incandescence appears. The bluish glow that appears before the electrodes glows yellow, is a theromionic discharge, and it is bluish because the phosphors produces mainly UVA, so we seen the original color of the discharge.

I do not know exactly which moment you mean (the player on my computer is responding slowly, so I couldn't catch the exact time).

Generally:
The thermionic emission starts at about or even a bit below 1000K, while that temperature is still too low for visible incandescent glow. If the filament is warming up evenly and/or if the emission coat is working along the whole filament length, it starts first the thermionic emission and only after that the incandescent glow (usually not visible anymore, unless heated to really high temperatures)
With many lamps there is a section of filament not covered by the coat anymore (wear, the way how the coat is applied,...), the naked filament reaches 2000K and starts to incandesce before the heavier coated part is able to reach the 1000K for the thermionic emission. With that you first see the incandescent glow of the naked part and only after that the thermionic emission appears.
Sometimes the naked tungsten reaches it's own thermionic emission temperature, but that is way above the level, where it is already glowing bright.

But all this needs some preheat time, so closed starter, and during that the middle section of the tube will be completely dark.
The case when the tube glows dimly and the filament surrounding by pinkish (the difference towards the center section is visible) discharge is really the cold cathode operation; happens just immediately after you click the switch, when the top tube is dark and only the bottom glows in that mode.

Thanks.
The starters that are used in the Bug-zapper, are Arlen EFS120, so they don't physically closes and opens, as they are electronic.