Reaction mechanism for accent development
Stage 1: Rhotic leaves idiolect. The environment here speaks a non-rhotic dialect. This stabilises the non-rhotic intermediate, just like a protic solvent stabilises SN1 intermediate.
Stage 2: The unstable non-rhotic intermediate has its vowel shift to a more backed vowel, which is more distinctive and this is more stable. Backing occurs.
Stage 3: Vowelophilic substitution begins and the syllable is attacked. New vowel replaces /u/.
Stage 4: The vowel becomes longer.
I don't think SN1 was the best metaphor but I guarantee I already lost most of you from stage 1. Potentially some other mechanisms are better so my Chem homies can suggest some.
Anyway let me know whether you agree with this reaction mechanism and why/why not.
(figure adapted from master organic chemistry)