Why would a hospital evacuate?

Immediate threat to patient/staff safety. At the other extreme are situations in which the event clearly causes an immediate life-threatening risk to patients and staff, and the hospital must be rapidly evacuated.

https://www.ahrq.gov/research/shuttered/hospevac4.html#:~:text=Immediate%20threat%20to%20patient%2Fstaff,hospital%20must%20be%20rapidly%20evacuated.