It may be that these effects help to preserve morpheme boundary and identity information. For instance, given the importance of derivation of verbs from nouns and adjectives using -Vna- and -ata-, shifted stress allows one to differentiate these morphemes from lexical -ata- (common enough) and those -na-'s that are part of lexical roots (also relatively common). -Vna itself will often lose stress and reduce to a tense vowel before other suffixes, leaving the shifting as a hint of its underlying presence.