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Emission Mix Runs Out
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The coating on the filaments/cathodes is called an emission mix and is necessary for the electrons to pass into the gas via thermionic emission in the at the fluorescent tube operating voltages used.
Over it’s operating life the mix is bombarded with electrons and mercury atoms during operation, but more than this, during the start-up process a larger amount is sputtered off when the tube is started with cold cathodes. This is why the control gear and which method of starting the tube it used has such a significant impact on how long the mix lasts.
Because of this process, if the lamp is typically used for less than 3 hours each time it’s turned on, the emission mix will normally run out before any other parts of the fluorescent lamp fail.
The dark marks that can be seen at the ends of older tubes is a result of the sputtered emission mix.
Once the emission mix has been sputtered off completely, the cathode is no longer capable of passing sufficient electrons into the gas fill to maintain the discharge at the voltage the tube was designed to use.
The control gear, ideally, would shut down the tube when it reaches this point.
There are some control gear that will provide enough increased voltage in this situation to operate the tube in cold cathode mode, but this causes overheating of the tube and therefore rapid disintegration of the electrodes and the support wires until such time as they are completely gone, or the glass cracks, which wrecks the low pressure gas ill and stops the gas discharge.