The area in Manassas where he shot is very highly trafficked. I agree with conjecture that he was p'ed off at the Baltimore Marine guy getting "his" (the sniper's) attention and decided to just show them who was really doing the shooting. Not to mention the fact that every dang report on local newsradio today was about how this was the longest the guy's gone without shooting since the whole thing's starting. The media reports have practically been BEGGING the guy to show his face (or rather, his gun) - and so he has obliged.
But this does feel, well, kind of hasty. The sniper's actions tonight, I mean. Since the Baltimore stuff broke kind of late, it seems the shooter scrambled to show his stuff tonight. I think he's on his way down the spiral and that he'll start getting sloppy due to rage at the police/media's behavior (I think that's already started, if the police have all these supposed leads), or else greedy enough to try for more than one shot at a time.