They're called Hunter-Killer groups because of the new tactic. Earlier destroyers would attack individually. With the H-K group they would spread out 1000 yards apart and sweep the area. Whichever one found the sub became the Hunter, slowing down to match the sub's speed and guiding the rest (the Killers) in for...well, the kill.
Carriers and aircraft were added later, when escort carriers started becoming available.
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