I just found this short-but-sweet explanation for why sleepwalking occurs by neurologist Antonio Oliviero. It appears in this month’s Scientific American Mind:
People can perform a variety of activities while asleep, from simply sitting up in bed to more complex behavior such as housecleaning or driving a car. Individuals in this trancelike state are difficult to rouse, and if awoken they are often confused and unaware of the events that have taken place. Sleepwalking most often occurs during childhood, perhaps because children spend more time in the “deep sleep” phase of slumber. Physical activity only happens during the non–rapid eye movement (NREM) cycle of deep sleep, which precedes the dreaming state of REM sleep.
Recently my team proposed a possible physiological mechanism underlying sleepwalking. During normal sleep the chemical messenger gamma-aminobutyric acid (GABA) acts as an inhibitor that stifles the activity of the brain’s motor system. In children the neurons that release this neurotransmitter are still developing and have not yet fully established a network of connections to keep motor activity under control. As a result, many kids have insufficient amounts of GABA, leaving their motor neurons capable of commanding the body to move even during sleep. In some, this inhibitory system may remain underdeveloped—or be rendered less effective by environmental factors—and sleepwalking can persist into adulthood.
As a bonus, the page also has an explanation of why we experience the painful ‘brain freeze’ sensation when we eat ice cream too quickly.
UPDATE: Thanks to Danielle for sending this fascinating snippet:
I used to have a VERY SEVERE sleepwalking problem. This past summer, I researched the use of GABA for mild anxiety. Although there was a great deal of question over whether it could cross the blood-brain barrier, I thought it was worth a try. It didn’t work for anxiety at all – but I was surprised to notice that it cured my sleepwalking, which was completely unexpected! Now that I know more about the connection between GABA, slow-wave sleep, & sleepwalking, it makes sense. I think there may be real treatment or research potential there, but I have no idea to whom I should report this. Maybe you can do something with it?
Link to SciAmMind sleepwalking and brain freeze explanations.
Mind Hacks 
Can people get suicidal from sleepwalking?
I am a 64 year old female. For the past 2 years I have had episodes of sleepwalking, and usually hear from other people what has transpired while I am sleepwalking. It has been very frustrating for me. I saw a neurologist, went through a sleep study, and still no answers. A few weeks ago my roommate saw me up and about and decided to call 911. It was a good thing that she did because I had a fever of 103, and had double pneumonia. What she thought was sleepwalking was actually delirum from my fever. During my hospital stay they kept testing my blood/oxygen level, and one night it had dropped to 78. Turns out that my sleepwalking was occuring because I was not getting enough oxygen at night. So, now I sleep with my oxygen on all night long, and am pleasantly surprised that there have been no more sleepwalking episodes.
I am 13 years old and I’ve always been sleepwalking ;/ It is very dangerous because se I know a boy who sleepwalks and actually got out of his house and got lost. No body knows what happened to him 😦 I always try to get out of the house while sleep walking..My father caught me trying to sneak out 😦