It's a term from literature/fiction, where later installments try to explain something from earlier installments; usually in a way that's nothing like the original intent. Applying this to real history is more like "revisionism".
"RETroactive CONtinuity", in addition to what the sibling comment said.
The Wikipedia top example is Sherlock Holmes dying in a fight with Moriarty and then coming back later when the author relented and decided to write more stories.