The Falcon 9's onboard computer probably detected a malfunction like a sudden loss of pressure in one of the first stage engines, which triggered an immediate shut down. With one engine gone, the flight computer recomputed a new ascent profile for an eight engine Falcon 9, instructing the remaining engines to burn 30 second longer to recover the thrust lost from the one engine's failure. So all's well that ends well. There weren't any major effects to the Dragon capsule or the cargo resupply mission as a whole.