It was not ruled a fumble. It was ruled an interception. The Team A player controlled the pass in the air, but did not maintain control long enough after returning to the ground to establish possession. A Team B player caught the ball (which was still a forward pass, not a fumble) for the interception. Simply touching the ground with control is not enough to have possession. The player must maintain control long enough to perform an act common to the game AND survive contact with the ground. He did neither of those things. If Team B had not caught the ball, it would have been an incomplete pass.