I have not heard that about 8' depth. Turnover can occur in any unaerated pond if the conditions exist. Cold rain can cause a plankton (source of O2)die off which will reduce O2 throughout the pond. Also debris washing in can cause available O2 to be used up in decomposition causing a fish kill. Cloudy days can also be a factor as plankton producing O2 is much less productive. Large temp swings can also cause problems for the same reason as cold rain. It sounds like several of these factors were at work in this event.