From my readings in this sub and also on /r/badhistory, it seems like the usual reasons(like Barbarians at the Gates) for the Fall of the (Western) Roman Empire are inaccurate. There are a ton of proposed reasons and the truth is probably a mixture.
However, what I am interested is the history of these reasons. What reasons would the historians in the past would have given?
I could have a crack at answering this, but I want to clarify with you. Are you interested in why earlier historians held the explanations for the Fall as they did (as in, the social or political environments that might have influenced them leaning towards a particular explanation) or do you want to understand how the earlier historians justified their explanations, the evidence they used and conclusions the drew from it?
As an aside, and I'm not at all an historian so please take this with a grain of salt (hopefully someone smart can back this up), I think the word you are looking for is historiography.