What causes enlarged/swollen saliva/parotid glands?

Many possibilities: See an oral surgeon or ENT physician for a diagnosis. Among other things it could be a blocked salivary gland or a tumor.

Various Issues: Glandular swellings, be they salivary or specifically parotid may be caused by systemic diseases of bacterial or viral origin: traumatic, metastatic or metabolic origins. The important issue is that no swelling should be overlooked and professional care should be sought to identify the etiology and propose a treatment. See your dentist or md for further information.