I have observed swirling mostly on brand new lamps and used lamps that have been put into storage for a long time.
IMO swirling occurs when the point of contact between the discharge and the filament is too small. This can be caused when the emitter is still in its initial state and the electrode is prone to building hot spots.
It also can occur when the filament is damaged or the emitter is worn out - this can often be observed in flyback-driven lamps (simple battery inverter style). They tend to swirl when the go EOL...
A small point of contact can also be caused when the lamp is mercury starved and the argon is the main carrier of the discharge current.
Swirling is certainly caused by impurities in the tube volume and is not related to cathodes behavior. It happens even in cold cathode lamps and even with non-activated electrodes like in old Soviet sign tubes.
Note that however because the tube is a closed volume at near vacuum, cathode problems can release impurities and this can start swirling.