No clue about the actual reliability of this[1] article but the mechanism mentioned (new pathways due to changes in crystalline structure due to uneven heating)

No clue about the actual reliability of this[1] article but the mechanism mentioned (new pathways due to changes in crystalline structure due to uneven heating) sounds possible. 1. https://incompliancemag.com/how-and-why-varistor-failure-occurs-including-the-effect-of-multipulse-surges/ https://incompliancemag.com/how-and-why-varistor-failure-occ... Reread your wondering and now conclude its about chained situations which this also does not answer.