Max pointed me to an interesting-looking book called Dangerous Creatures of Africa by Chris and Mathilde Stuart, which he recommends. An excerpt:
Jackal hunter Peter Schneekluth was tracking black-backed jackal on a farm in the Karoo when he noticed a male ostrich moving towards him. It rapidly closed in on him, then leaped into the air and kicked out at him, knocking him to the ground and kicking at his back as he lay. Peter managed to grasp the bird in a 'neck-lock' while holding on to one of its wings. Both combatants whirled and then crashed to the ground. Noting that the ostrich was becoming limp, Peter slackened his grip, but the bird attacked more fiercely than ever and he was forced to strangle the ostrich. A heavy leather belt Peter was wearing was cut neatly in half --- probably what saved him from serious injury in the initial attack. He escaped with extensive bruising and minor cuts.
A territorial attack, perhaps?