I believe that all party´s at BHD have to agree 2 keep sufficent staff on to cover a delayed arrival after 21:30. Organisations such as Fire and Rescue, Security, ATC, Handling agents, and other relevent airport departments. If there´s more than 1 flight delayed after the 21:30 deadline then it´s more likely the airport will remain open.
I may be wrong but thats what i´ve been told my different people. Maybe someone can confirm this!