What are the causes of swollen lymph nodes in the neck?

Many things: There are many causes of cervical adenopathy. It occurs during many upper respiratory viral and bacterial infections, mononucleosis, CMV infection, bacterial infections of the face, scalp or neck etc.

Monitoring Centers: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells and digested food. Lymph nodes "taste" the lymph for anything bad like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.

Usually inflammatioi: Usually swollen cervical nodes are caused by an infectioin in the skin above the nodal chain. It could also be a systemic infection or even a lymphoma. You shoud be seen by your physician.