One could argue that a certain style of historical fiction, perhaps kicked off by I, Claudius and hovering in the space between Robert Graves and Georgette Heyer, has never gone out of fashion and indeed includes many best sellers of the last thirty years.
Pipes and Timbrels delved a little into this apparent rebirth in a recent edition, citing Shakespearean scholar Martha Tuck Rozett's essay "Constructing a world: how postmodern historical fiction reimagines the past", on the latest generation of historical fiction writers:
What they share with the new historicists - and what distinguishes their novels from traditional or classic historical fictions and allies them with postmodern fictions - are a resistance to old certainties about what happened and why; a recognition of the subjectivity, the uncertainty, the multiplicity of truths inherent in any account of past events, and a disjunctive, self-conscious narrative, frequently produced by eccentric and/or multiple narrating voices.Clearly this applies more to Geraldine Brooks and Julian Barnes than to Bernard Cornwell, but writers such as EL Doctorow and Laurence Durrell were doing interesting and new things with modern history way before anyone barthed up (sorry) post-modernism.
I'd suggest that the new directions in history writing in general (fact or fiction) are based more in post-war and more specifically post-Cold War cultural uncertainty, greater access to archival information, the impact of modern history and particularly war, influences from fields as diverse as psychology and forensics, broader international dialogue between writers and researchers in history and archaeology, and new forms of writing that pre-date post-modernist theory.
That, too, has been a recent influence, but it is not the only nor the most significant one.
It's all part of the same wave, if you like, currents heading off in different directions, overlapping and clashing and sometimes drowning each other out.
I just made that up, but I'll call it a theory.