The 簸 reading is the everyday one: the verb 'to winnow', and the common word 颠簸 (to jolt, bump along). The separate 簸 reading appears only in the noun 簸箕 (dustpan).
Under the bamboo top, 其 is the old picture of a winnowing basket (it is the original graph behind 箕), tying the character directly to the act of winnowing.
