I don't actually have a definite answer, but my boyfriend is going through the same thing. He had a very serious brain injury and has of course lost is hair where he had his surgery. However, he had two sores on the back of his head (bed sores? I'm not sure) that nobody noticed until he was at the rehabiliation center (1 month after his injury) when his hair started falling out, but just in those strange spots in the back of his head.
He hit is head on the right side, and also received two bruises in his frontal lobes from his brain bouncing when he hit his head. The doctors never said anything about him having any kind of injury in the back. There was swelling of the brain and it can continue to swell for up to 3 months, so that may have something to do with it, if the swelling is severe enough. That though, is entirely a guess..the doctors haven't said anything about that to me.
It is starting to grow back a little in some places, but it's also started to spread just a bit (not enough for him to notice). I read somewhere that after a serious illness or high fever (which often accompanies head injuries) hair can fall out. It says a bit more on this web site, near the bottom, but I have been unable to find much more info. I hope someone will have a better answer soon! [removed]
One last thing, I have heard of kids who lost their hair due to a brain injury/head injury and needed hair plugs. So I guess it just depends if it will grow back.