The syllables are not arbitrary but were due to a scribal error. The Masoretic text of the Hebrew Bible has a system of indicating vowels through pointing added around the text. In some instances, where the text contains an error the Masoretic scribes would put the vowels of the word you were supposed to say instead of what is written. For example if the word ידו ydw is found where ידי ydy should be, it will be pointed יַדִו y(a)d(i)w to indicate the word that should be pronounced instead. This happens with the Tetragrammaton: the vowels around יהוה YHWH are the vowels of אֲדוֹנָי ’Adonai the word that is supposed to be read instead. That means the Name appeared as יְהוָֹה Y(e)h(o)w(a)h, leading a hapless medieval scribe to transliterate the word as Iehovah. (J is a late Latin innovation for an i/y glide that begins a word.)