This is just a hunch... If you look at the pattern, we had two days of multiple shootings in the northern suburbs. The evening of the first day, there was publicity, but they hadn't yet determined that the two shootings were connected.
On day 2, he shot 4 people in the morning, and after that, the was no doubt that all these shootings seemed to be tied together. At that point, a very disorganized manhunt was beginning. He had finally gotten their attention. And just to drive the point home, he went out and shot a guy in downtown DC that evening.
Now, he's watching FNC every time he's home, because they are the "All-Sniper, All the Time" network this week. He knows that the authorities are mustering in Montgomery County to begin a serious hunt for him. There are roadblocks beginning. So he decides, just to throw them off, to go off in a completely new direction - many miles to the southwest, far away from the previous shootings, yet still easily accessible by freeway. Because he knows that the search is concentrated in the northwest near suburbs. That's why he chose Fredricksburg, I think.
After that, he purposely bounces around rather than conform to an easily identifiable pattern. He modifies his MO based upon the intelligence he gathers from the news.
Probably because his early multiple shootings were done in an information vacuum. The first day, all they knew was the had a couple of shootings in the DC area, nothing all that unusual about that. After the second day's carnage, he knew that the element of surprise was gone, so he couldn't risk multiple shootings, and he had to take his handiwork someplace he hadn't been before, because Montgomery County was too hot.