Many Oxalis species that have bulbs produce an underground stem with roots. This stem is the first to appear as a thin white thread, not the roots, when the bulb spourts in autumn. This is why it is safer to plant the bulb on the side when it is not clear which is the right side up. By planting shallowly you are forcing the bulb to not produce that natural roooting stem. The deeper the bulbs, the more stem roots they produce , which is of course, good. Something similar is commonly seen in Lilium plants. It is a natural mechanism in many bulbs that a bulb neck deep in the ground is able to support a top heavy flower/flowers.