As we have yet to hear an official reason from SpaceX these are just PURE SPECULATION at this point.
1) The International Docking Adapter or a part of it came lose during the flight and impacted with the top of the 2nd Stage. This caused damage to the 2nd stage that resulted in the explosion.
2) The Liquid Oxygen tank ruptured causing the explosion, either due to a manufacturing issue or because of a leak in the Helium Pressure system which caused too much pressure to build in the tank.
At this point it is impossible to say if these are even close to the truth and until we hear officially from SpaceX the speculation will continue to grow.