> Must sound change be linguistically motivated?
Of course not! We know that some sound changes are socially motivated, for
example, social climbing, or group identity. So when young people today say
/?/ instead of their parents' /t/, it is about street cred, not phonetics.
Similar explanations have been offered (rightly or wrongly) for the Spanish
lisp - it was imitation of the speech patterns of a superior. There are
many such examples, from grammar as well as phonetics.
Peter