Normally, when a massive star reaches the end of its life, its core collapses into a single black hole. But if the star was spinning very rapidly, its core might stretch into a dumbbell shape and fragment into two clumps, each forming its own black hole. A very massive star as needed here often forms out of the merger of two smaller stars. 

• And since the stars would have revolved around each other faster and faster as they spiraled together, 
• the resulting merged star would be expected to spin very quickly.