It happens to me too. Home is the place where I stutter most, hands down. It isn't even remotely close. I have a theory for this. They say that adrenaline in the blood actually lowers stuttering. Have you ever noticed that when you're in a important argument, moment of panic, or moment of intensity you tend to stutter less? It because somehow adrenaline makes us speak more fluently. Where am I going with this? Your home is where you have the lowest concentration of adrenaline in your blood. You're super comfortable with the people around you and the environment is very familiar. This causes you to have lower adrenaline and stutter more. I've literally always wondered this too and I researched the answer recently. What upsets me is that my family probably doesn't realize who I am in the outside world...they think I'm just as unsmooth inside and I am out with others lol. This hurts my soul a little.