A good example of of why not to open carry, makes it even easier for a gun grab.

Never let anyone get within your reaction gap which is about 4 feet and never turn your back on someone close when carrying.
You can usually see a gun grab attempt coming if you watch peoples eyes. They will look at your waist line to try and visually locate it before they strike. If someone grabs you like that, the best defense is to gouge their eyes out. Your lucky.