It's not a myth, and has been seen reliable witnesses. Its mostly likely cause is that occasionally, ionization-induced plasma outputs RF that by accidentally matching of volumetric parameters (size) creates a self-sustaining RF discharge, where the RF continues to maintain the plasma and vice versa. A circular mechanism. Obvious it cannot last forever, but the plasma has enough stored energy to keep the cycle up for awhile. As a charged ball it may be affected by earth magnetic fields and then it moves. At some point the stored energy, through energy losses, drops below the minimum needed, and the ball self-destructs by expending its total energy rapidly. Tesla accidentally produced such balls, but repeatably, identifying that it is some arcane physics phenomenon and not myth. My personal guess is that it requires not just gasses but some seeding chemical, maybe dust of particular kind.