It's unknown. Lipomas are extremely common, benign fatty tumors that typically appear on the truck, arms and or thighs. They are typically slow growing, soft lumps just under the skin that appear in middle age. The cause is not really known, although there may be a familial predisposition. The only way to get rid of one is by surgery, but often they are just left alone.
Genetics or trauma. Usually genetics or old trauma.