Ancient Quenya Phonetics P28: [j], [w] often became [i], [u] after consonant groups

AQ. [j], [w] often became [i], [u] after consonant groups; [CC{jw}|C{td}j|C{kg}w] > [CC{iu}|C{td}j|C{kg}w]

Primitive Elvish allows for tri-consonantal combinations only when those combinations end in a y or w. In many of those clusters, the final y, w developed into i, u, as described in the Outline of Phonetic Development [OP1] from the 1930s and the Outline of Phonology [OP2] from the 1950s:

Ancient Quenya Phonetics P21: syllabic nasals developed a preceding vowel of similar quality

AQ. syllabic nasals developed a preceding vowel of similar quality; [ṃb-|ṇd-|ŋ̣g-|ŋ̣gw-|ŋ̣gj-] > [umb-|and-|iŋg-|uŋgw-|iŋgj-]

Primitive Elvish words allowed for initial nasal-stop clusters (PE18/43-44, 93), and in some cases those initial groups became syllabic. The exact conditions under which this syllabification occurred isn’t entirely clear, though sometime in the last 1950s or early 1960s Tolkien said “but in some cases, usually words of strong or emotional meaning or important names, the nasal became syllabic” (PE17/124). In the 1930s he wrote: