According to the Mayo Clinic, 80% of people with restless leg syndrome also experience a condition called Periodic Limb Movement Disorder (PLMD). PLMD is defined as the repetitive cramping or jerking of the legs and/or arms. Interestingly, it is the only movement disorder that occurs during sleep. While many people with RLS have PLMD, only a minority of people with PLMD also have RLS. There are several other conditions that seem to be correlated to restless leg syndrome, but as of yet, no one knows why.
Link to Mayo Clinic