In such a case, where all the events in question occurred in your home state and the guy suing you was from another state, you'd get the benefit of your home state's substantive law.
Now, in the case of the rock thrown into Virginia from Maryland, your actions did harm in a different state, so you're not really doing eveything in the borders of your own state- the damage arose in Virginia.
I can see that to a certain extent, but mostly because a rock is a pretty, uh, concrete example. But some states might consider actionable things that other states wouldn't think to. For example, laws against (so-called) excessive noise, or (so-called) pollutants, or whatever. I could be doing something in my state that's perfectly legal here, but someone over the border could object and claim that I caused harm to him. And in that instance, it seems I'd be subject to the laws of his state.