Sternberg, E., Lee, G.C., and Huard, D. (2004). Counting Crises: US Hospital Evacuations, 1971-1999. (Abstract only.) Prehospital and Disaster Medicine. 19(2):150-7.
The authors reviewed various types of articles to determine causes for hospital evacuations and found that more than half were due to hazards originating inside of the facility or from "human intruders." They emphasize the need for data gathering and reporting to facilitate the calculation of national data.