I volunteer with a rescue squad, and we are required to have overnight stays. We have ALS, BLS and heavy rescue units as well as the fire department support of engines, trucks, etc...
Many time, we get calls in the night(sometimes , not to the best areas)and we go alone, unless there is a need for "manpower". This means that certain types of calls(high rise buildings, car accidents, mass casualties)require you to have more rescuers than you might have with you. (and they can walk all the way back to the unit to get the stuff too!)
Units call always request extra manpower when they get there, but the dispachers usually know what calls will need them beforehand. This would be the only reason I could see that they would all get called out.