It's usually caused by surgical removal of the lymph nodes in axilla (node dissection). Radiation can make it worse.

Lymphedema generally occurs when some lymph nodes are removed like when you have a sentinel nodes biopsy or axillary node dissection. Radiation alone without lymph node removal can also cause it but this is more rare.

While radiation field size does increase your risk of lymphedema slightly over surgery alone, the type of surgery you had is the biggest risk factor by far. Radiation certainly doesn't make matters better, but surgery is usually the culprit.