The common marine ich (Cryptocaryon irritans) and the so-called black ich (either Paravortex or Ichthyophaga species) are both parasites. While stress can certainly impact how bad the outbreak is, the parasites must be present. All the stress in the world cannot make a parasite spontaneously appear.
Those instances in which it is alleged that there were no new additions, something happens, and all the fish got ich are easy enough to explain. For one, perhaps the parasite was introduced with something other than a fish. Very few people quarantine their corals or mobile invertebrates, even though they should. Secondly, the infestation could have been there and just gone unnoticed. The bugs could have been in small numbers due to partial immunity or in the gills where they went unseen. And lastly, sometimes people just lie. Maybe they don't want to admit adding a fish recently that was infected and now all their fish are dying.