It's because when monkeys descend a tree they do a kind of loosen and slide, loosen and slide down, but the babies haven't figured that part out yet and they'll try to climb down the same way as up, they'll try and turn their whole body round sometimes to climb headfirst DOWN the tree but gravity just causes them to fall. That's why they only climb high trees with their mothers or she'll just stop them when they try to climb too high on their own.