Have seen tubes do this on startup in cold environments. My best guess is reduced mercury vapor pressure in the tube, because it will often go away after the tube warms up a bit (as long as the environment isn't TOO cold). Some newer tubes seem prone to this, perhaps due to the lower initial Hg content.

A very neat looking effect. For more fun, you can "drag the arcs around" using a strong magnet. [Linked Image]