The main causes of sliding masses in the back of the neck include benign body surface masses and calcification of the collateral ligament. 1. Benign superficial masses: lipomas and sebaceous cysts. Lipomas are associated with an abnormal accumulation of fat in the nape of the neck, while sebaceous cysts are usually associated with an abnormal blockage of the sebaceous ducts in the nape of the neck, which results in an excessive accumulation of sebum that cannot be drained. All of these benign body masses present as sliding masses in the back of the neck. 2. Calcification of the collateral ligament: Calcification of the collateral ligament of the cervical spine is caused by calcium deposits within this ligament. Chronic strain injury to the soft tissues of the neck, such as working with the head down for a long period of time and playing with cell phones, can cause degenerative changes in the cervical spine, resulting in calcification of the cervical collateral ligament and calcium salt deposition, which will also lead to lumps in the back of the neck in the long run.