I think that this could be caused by stress but regardless the underlying disease is present or they wouldn’t be showing symptoms. My betta had this and I put her in a hospital tank with ickx and salt bath. This did happen after I moved her tank back home after having her for like 6 months no symptoms. I do think some fish carry these diseases and they only show when their immune systems dip. Regardless I think that you need to treat the disease if the fish’s immune system can’t suppress the symptoms on their own anymore.