Personally, I don't think anything conclusive has ever been discovered, nor will there ever be. I don't think there is enough consistency to be able to make a control group for the investigation. No one particular thing triggers this, it can happen at any age, circumstances are to wide ranging. Altogether to many inconsistencies and variables.
We can only surmise or theorise as best we can. It could be chemically activated in the mind (not by drug induction), or it could be a future memory, or a past regression and so on. Or maybe the matrix has us.
Perhaps one day it will be explained but not at the moment.