Sounds like a mild case of "lawnmower syndrome" (you can search on that
term, below, to the right). I'm thinking that when the car starts it
runs rich (all cars do to ease starting). If you then shut it off
after a second or two there might be a lot of unburnt fuel in the
intake manifold. If it does not evaporate(why, I don't know) then the
next morning when you start the car it gives an extra dose of fuel (as
normal for a cold start) on top of the fuel already sitting in
the intake causing an over rich condition. I suppose this scenario could
also wet the plugs or coat the cylinder walls with fuel (which reduces
compression making the engine hard to start). This is all just a
"theory" / "wild ass guess" / "its the internet so we are all experts"
sort of thought (I'm not expert).

My wife has a 1995 850 and with that car I never run it less then 60
seconds to avoid this kind of problem (on two occassions I have had
it flood and not start after running it for just a few seconds). I
have a 98 S70 and have not had any starting issues of this type with it.