The less frequent occurrence of o in several dialects is linked to the change of o into u in non-initial syllables, and in the Eastern dialect, to the unrounding of the o in the first syllable, i.
Do the differences in the spread of o point to a more general tendency of rounding in the western dialects of Estonian and unrounding in the eastern dialects?
As an example of unrounding is the change o > o in the Eastern dialects, as in e.
64-78): NHG diphthongization and monophthongization (the latter idiosyncratically called 'Mitteldeutsche Monophthongierung'), lowering of high vowels, quantity changes, rounding, unrounding
, and contraction.
Forward-shifting of /uw/ and /ow/ is also true of the Southern shift, except that in all three cases it is not so much a matter of shifting as of unrounding of the first element of the diphthong, with the Southerners having gone furthest.
The centralizing of /uw/ and /ow/ are really simply unrounding of the first segment of the diphthong, thereby spreading the distance between the starting point and the finishing point, i.
This tendency toward unrounding is so strong in American English that one variety of American English, now probably the majority variety, from the Plains through the Rockies and the West Coast, close to a "standard" pronunciation followed by more speakers than any other variety in the English-speaking world, simply has no round vowels at all.
But the change of "short u" to [LAMBDA] is simply unrounding, with the apparent "falling" being predicted by any theory of marking, preference, or optimality: namely, elimination of the highly marked vowel that results from unrounding of short u, that is disfavoring of [u] (Round) or [i] (Unround).