From WebMD:

Several things can lead to sleepwalking.

It can run in the family. Identical twins are more likely to sleepwalk. If you have a parent, brother, or sister who sleepwalks, you're 10 times more likely to do so than someone from a family with no sleepwalkers.

You might also have the disorder if you're:

Sleep deprived
On a chaotic sleep schedule
Stressed
Drunk
Taking drugs such as sedative-hypnotics (which promote relaxation or sleep), neuroleptics (used to treat psychosis), stimulants (which boost activity), and antihistamines (used to treat symptoms of allergy)
Medical conditions that have been linked to sleepwalking include:

Heart rhythm problems
Fever
Heartburn
Nighttime asthma
Nighttime seizures
Obstructive sleep apnea (a condition in which you briefly stop breathing during sleep)
Restless leg syndrome
Psychiatric disorders, for example, posttraumatic stress disorder, panic attacks, or dissociative states, such as multiple personality disorder