How often do members of Congress resign in the middle of their terms, without something forcing them to do so?
So the really interesting question in all of this is - what is that thing that is forcing him to resign? Is it connected in any way with the other big story of the moment, namely all the Russia stuff? Or something less serious and maybe only personal/family? The more benign explanation is that there could be a health issue of some sort - but usually when that is the case people say so.
I think he thought Clinton was going to win, and was planning a frenzy of investigations and public hearings. Now that he doesn't get to have his fun, he wants to resign.