Don't know why but every time I see a fluorescent light that does what the question asks i get really happy and just stare at it to see what will happen next
Even better are the striation that seem to swirl, the striations that are hard to see because of the speed of them, and large striations
Other effects are Mercury Migration and Striations with MM
Who else is fascinated with these effects including EOL

Because it is not so frequent sight, unlike the boring properly working tubes... ;-)

For me I bet it's just an ADD thing.  We get fixated on strange performing things and hyper focus on it, trying to figure out what, where, why, and how.