Definition of 'possession' by a goalkeeper
"Once the goalkeeper has gained possession (also known as “control”) of the ball, an opponent may not interfere with or block the goalkeeper’s distribution of the ball. For example, players have a right to maintain a position achieved during the normal course of play, but they may not try to block the goalkeeper’s movement while he or she is holding the ball and trying to distribute it. Nor may opposing players do anything to hinder, interfere with, or block a goalkeeper who is throwing or punting the ball back into play. The goalkeeper has already gained possession and is granted up to six seconds to release the ball back into play by other players. A goalkeeper in the act of distributing the ball may not be challenged under these circumstances. (This includes trying to head a ball out of the goalkeeper’s open hand or playing a ball being bounced or tossed into the air by the goalkeeper.) An opponent does not violate the Law, however, if that player takes advantage of a ball clearly released by the goalkeeper directly to him or her, in his or her direction, or deflecting off him or her nonviolently”
In yesterday's case, Rushworth definitely had 'possession' and 'control' of the ball with both hands but it was dislodged from his 'possession' by Ennis's head as he stood up. Yes, it was clearly unintentional but that doesn't matter, any infringement is still an infringement whether intentional or unintentional. In fact, free-kicks are probably awarded more often for unintentional fouls or handballs than for clearly intentional ones with the latter more likely to result in a yellow card. Maybe it was borderline at worst but reading that, I think the ref just about got it right.