People overexplain things constantly, regardless of gender. And quite often they don't know what they're talking about. I'm a college professor with a doctorate and teach in two different fields, but I can't begin to tell you how many times I've had students (both men and women) convinced they know more about the subject I'm teaching than I do. Quite often, the scenario begins with my introduction to a topic (that I intend to build upon to much deeper levels that required those years of education and research to explore), but because the introductory material is something they've been introduced to before at some point, they'll stop listening to the rest of the conversation and want to start refuting simplistic information (not realizing that such information is completely irrelevant to what they need to learn). I suspect a lot of "mansplaining" is tied up in this phenomenon, where someone knows a little bit about something and wants to blurt out what they know before the original speaker even gets to the point she was trying to explore.

I once watched a young captain interrupt a seasoned veteran's speech about the invasion of Panama to "correct" him on what occurred during that invasion, even though he was addressing the former colonel who led the infantry assault that the captain had only read about.