You need a few ions around in the Neon for the cascade to start. This can happen for a variety of reasons including cosmic rays, background radiation or doping of the gas mixture with something radioactive. There is at least one thread on here about radioactive nixies (which sent me off on a side quest that hasn't quite finished yet!). Apparently 5092's are dosed with Kr-85 and have a little radioactivity symbol on them.
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