Every language has what are called morpheme structure rules. These are generalizations about what combinations of sounds are permitted in certain positions––initial, medial, and final. Some clusters of sounds that are permitted across morpheme and word boundaries are not permitted in word-initial or word-final position. Exceptions may be restricted to foreign borrowings. In English, accordingly, the combination ts- only occurs word-initially in borrowings such as tsetse fly. But because this cluster does not conform to native morpheme structure in word-initial position, it tends to get simplified in the speech of persons who cannot or choose not to make the phonetic effort to pronounce an unusual cluster, and the first consonant is elided in a process that is called by the technical term APHAERESIS. Thus ts > s.
A contemporary illustration of aphaeresis in English is provided by the word tsunami ‘tidal wave’, which has been borrowed from Japanese. While the initial cluster ts- is completely regular in the donor language, it contravenes the rules of English, thereby lending itself to simplification as [sunámi], a pronunciation commonly heard these days––alongside the non-aphaeretic [tsunámi]––with reference to the recent catastrophe in Japan. Why the second member of the cluster and not the first is elided can be accounted for by the general rule pertaining to initial clusters in English whereby it is always the first and not the second member in this position, as in the pronunciation of words like knight, gnat, psychology, phthisis, etc., whose Old English pronunciation, resp. that in the donor language, involves an initial (unsimplified) cluster.