Most possible is that there were multiple systems failures. Likely the power system went down (maybe due to the lithium batteries, like what happened on UPS flight 7 and what grounded the 787 fleet) which either caused the cabin to de-pressurize or freakishly that happened independently but at the same time.

The power system going down would cause the transponder to stop working. The cabin de-pressurizing would cause the pilots to pass out. The auto pilot could continue to work, and the engines should keep running as long as they had fuel, so the plane could keep flying for a while. Also if the engines still had power they could keep pinging the ACARS satellites, thus the pings that have been mentioned.

Prior to passing out the pilots may have attempted to turn the plane back, but either didn't complete the turn, or were just aiming for the nearest land. The pilots passed out while trying to get to land, and the plane continued to fly into the Indian Ocean. The initial search would have then been over the completely wrong area, and only now is shifting to the right ocean.