I understand that hypostatic pneumonia can be caused by a virus or bacteria (if a patient is non-ambulatory for a long time). are there any other causes?

Interesting: Hypostatic pneumonia usually results from the collection of fluid in the back part of the lung especially in bedridden or elderly patient confined to a supine position for an extended period of time. There is a good chance that this makes the patient susceptible to on going infection. Close observation by a pulmonologist should be sought.

Yes: Fungal in some cases as well but very unlikely unless immunosuppressed. More likely bacterial or viral. Depending on the facility you may be able to get chest physiotherapy which may assist and possibly reduce the risk of this unfortunate complication developing

Pneumonia: means inflammation of the lung tissues, there are several causes for it, hypostatic, aspiration, etc.. denote the cause/mechanism by which the pneumonia developed. In case of hypostatic pneumonia, prolonged recumbancy in a certain position in weak debilitated patients leads to not enough ventilation of the lungs in a certain parts & stagnation of secretions which can invite infection of any kind