What causes someone to have acute delirium?

Depends. We believe delirium occurs when the brain cannot maintain its normal activities when overwhelmed by metabolic demands or other variables which exceed the brain's capacity to compensate for. Brain with less reserve capacity are more vulnerable. Not entirely unlike acute heart failure. This is different than delirium tremens which is caused by neurochemical changes from drug/etoh withdrawl.
An illness. Delirium is a syndrome that involves confusion, poor attention, and other symptoms. It is most often seen in seeverely ill people who are hospitalized. Drug use and drug withdrawl can also cause it. People with dementia and other brain illnesses are at highest risk.