Even CrossFit isn't completely random, there are limits to what you do in a CrossFit workout. I'm sure over time you can adapt to CrossFit as well. It's just easier to overtrain with CrossFit because the volume is high, there are a variety of movements, and they encourage you working until you puke.
You could overtrain with even a a single movement like squats if you upped the volume quickly enough. Take someone who doesn't squat and have them do 10x3, 5x5, 3x10, 5x20, and 200 air squats 7 days a week. They'd probably get tendinitis before they burned out, but I guess that's another way CrossFit wears on the CNS more, the variety of movements allow you to work around tendinitis in certain joints to keep blasting yourself.