 I'm not sure. Phonetically  is usually a labialized postalveolar approximant . The labialization could cause vowel changes, like the development of a labialized velar approximant at the end of the diphthong. Though it's odd, because there's an unrounded vowel directly after the vowel in , which would maybe not usually be rounded. I guess the reason why would depend on the precise phonetic change: I'm not quite sure what it is, the change of  from nucleus to edge of the syllable, from full vowel to semivowel, the change of near-close  to close , or the development of final ? Hard question. — ·   