That comes from the Varronian Chronology, in which he lists those years as not having any consuls or tribunes. If we are to take it as true, the presumption would be there was great instability, perhaps stemming from unrest by the plebians, which would explain why the Lex Licinia Sextia (which guaranteed 1 consul would be a plebian) was introduced around this time, though not in effect until 367 BC. But that's not necessarily a safe assumption to make; there is some doubt as to whether there really were any "anarchy years". When Forsythe mentions those years he frames it as "allegedly", but I'm not an expert on the historiography of the Chronology.