Tanafur al-kalimat arises from the sequence of several words even if each word is individually acceptable. The transition across word boundaries may become cumbersome for articulation.
Unlike tanafur al-huruf, which is located within one word, this is a defect of the assembled expression. A sequence of similar sounds at word boundaries may create excessive burden.
Alliteration and repetition, however, are not automatically defective. The rhetorical judgment depends on whether the combination actually becomes unusually heavy.