Sinsear travelled from Cable's future to battle Cable in the present. After being defeated Sinsear decided that he needed his humanity back, which he believed he could only get by going back to the future. Sinsear broke into Greymalkin's Time Displacement Core (TDC), which was located at the bottom of the ocean. Cable & Bishop tried to stop Sinsear and a fire fight broke out, during which the TDC was critically damaged.
Sinsear relised that his humanity was not in the future, but inside himself. To stop the TDC exploding and rearranging time, Sinsear sacrificed himself.