May be traumatic implantation or part of systemic disease.
Many cases are sterile and idiopathic.
Can be part of a systemic disorder if multifocal. Sometimes part of pancreatitis due to systemic release of lipase. Autoimmune disease such as SLE can result in inflammation of the subcutis. Also can occur secondary to a vasculopathy. Must rule out infectious disease and the above listed systemic disorders before deciding it is idiopathic.