I think it is related to the physics of the mouth producing the sound, and we do a form of synesthesia: doing b-u-b seems (to me) quite smoother of a transition than k-i-k. If I stop blowing the u sound my lips close again; when I finish the i I have released the muscles and I need to hold again for the next k. It al feels more sudden an explosive with k. Also the b sound you voice it (otherwise it would be p).
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