It's because they are dark.
Once this process is started, can it be reversed? Realistically, can this sort of blight be turned back?
No. A wise and prudent person knows that this kind of illness is only cured by death. People who don't understand what is being talked about may tell you otherwise. Vault dwellers understand that death for a civilization doesn't necessarily imply the death of an individual. The whole point is for individual survivors, the "remnant," to go through the cycle and come out the other side carrying the values of that civilization forward. The bloom is followed by the nadir, then the decline and the collapse. The ants are supposed to survive when the grasshoppers die as winter approaches. This is the reason that the human spectrum includes ant-like personality types. That is what they are supposed to do.
If it smells like a Depression, looks like a Depression, feels like a Depression, sounds like a Depression, then it's a Depression. Not a "cyclic recession" or "slow starting recovery." Modern people are insane. They think if they all take a vote and get a consensus then they can decide this isn't a Depression, it's actually a boom. They have become too stupid to continue living. You do not pull out of a Depression by pretending it's not a Depression. You first have to recognize the problem correctly, then you can come up with the solution. Anything else is the pathetic flailing of lunatics and similar to the hysterical thrashing of a man who finds himself in quicksand. You'll only make it worse with all that vain unfocused struggling.