Its a developmental abnormality...one of the hypotheses is that there is abnormality in CSF flow(obstruction/pressure) which leads to cavity formation...this is supported by its association with arnold chiari malformation(More than 50% of these cases have Syringomyelia)...cavity formation can also occur after trauma, due to some tumors...
This is commonly seen in cervical region...I dont know why here only...i will try getting more info..