Lymph nodes are the filters given by nature they filter out some antigens,bacteria or viruses. when these antigens logged into lymph nodes, lymph node try to destroy them and due to this there is inflammation and increase in size of lymph nodes.  every lymph node drains a particular area so your baby may have some minor trauma or injury or infection at draining area that's why your baby may frequently have on and off swelling behind  ears due to lymph node swelling