I can't think of another bombing case with this many attacks in such close proximity. They usually go quiet for a while, which as you said, would be advantageous to him. The fact that there is no clear pattern in terms of victimology, is certainly making the job of law enforcement that much more difficult. It could still of course be race related, but the fact that the last two victims were white, lessens that possibility somewhat.
These seem to be pressure devices, tension causes the device to be triggered. The Unabomber built devices this way, where someone moving the device triggers it. He also rigged packages to be detonated when they were opened, once again using tension to trigger the blast.
I agree. Bombers are usually very intelligent. It's almost certainly a man, and likely white (most bombers are). He has to build these devices in privacy, so you're likely right about him not having a girlfriend and being a loner. Certainly arrogant, and enjoying the attention. The police are trying to draw him out, appealing to this arrogance and hoping that he will begin communication.
As far as why they exploded when they did, jmho it was the way set up. Think of all the movies we see (I know just a movie but for a visual) there are some that the people have to be very still because of the trigger on the bomb moving the bomb will cause the activation. That seems to be what happened with the package bombs.
