Hello,
Most lumps in that site(behind ear) are due to infections or benign tumors of soft tissue origin.
Behind the ear there is skin, fat,
cartilage, bone, nerves, blood vessels, lymph nodes, and salivary gland tissue (a portion of the parotid salivary gland extends behind the ear). A bump can develop from any of those structures, and could be due to a tumor (benign or not), an infection (Lymph node or skin), a swollen muscle or salivary gland, etc.