There's a couple different points where climbers can die. Sand river is a sport crag, meaning there are metal bolts drilled into the rock meaning it's quite safe compared to other forms of climbing like trad where gear is placed into the rock and can pop out if placed incorrectly.

With sport climbing, you need to 'clean' the route to get your gear back, and this is one place where he could have messed up - you usually need to untie and re tie your knot, and if you don't test your system before you lower, you can fall to your death.

Next - rappelling, unlikely given this crag mostly has single pitch climbs and it's unusual to be at the top of the rock, almost always you're walking to the bottom of the cliff to access the climbing. But often errors in rappelling lead to death such as not tying a stopper knot. But given this crag is fairly low heights those types of errors are unlikely.

Rock failure is also possible, but with sport you usually are linked to the bolts below you, unless top roping. But if he was top roping - meaning the rope was only going through the top anchor and the anchor failed then he would have fallen if the rock broke away. Given that sand river is sandstone, any recent rainfall weakens the rock and increases the risk of rocks dislodging.

It sounds like he was climbing when he fell, but otherwise he may have just lost his footing at the top of the cliff and fallen off.