FWIW, IMO it sounds more like a bacterial disease rather than a parasite. Could be wrong.
It's probably not an issue of dormancy so much as it is of opportunity. Bacteria that are perfectly harmless when they are competing for survival with other bacteria in a diverse ecosystem can become virulent pathogens if they become the predominant species or strain.
A rapid change in pH or temperature, or change in chemistry (medications or even the mist from spray can air fresheners landing in the tank - especially the "air sanitizers") can be the triggers that cause such a disruption to a bacterial ecosystem.
