Roseate Spoonbills usually call out loudly when they want their flock to leave. It’s a deep coughing sound, then the group takes off.
The first three photographs here show three different Spoonbill calling out to leave. However, none of the others paid attention.
If multiple birds signal it’s time to exit they should have. I guess they didn’t feel any threats. Still, it would have been prudent to leave.
The Snowy Egret just did what all the others wanted. I assume he didn’t speak Spoonbill.