What causes cervical lymph nodes to be reactive?

Many things: Lymph nodes enlarge when they are doing their job properly, fighting inflammation. This could be a viral or bacterial or fungal infection, reaction to dental or gingival disease, or less often, malignancies. Chains of nodes drain from different areas and it might help to know whether this is behind or in front of the muscle running diagonally from the jay angle to the collarbone.

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.