I don't know anything about Spanish language changes, but a change needn't have the entire population afflicted in order to occur-just enough people for it to become fashionable. And as modern trends constantly demonstrate, fashionable trends can come from anywhere, no matter how small a section of of a population, nor how silly the trend seems to be to the population at large, even as the population at large is overtaken by the fashion.
Even centuries later (1200s) the king Alfonso X the wise had to choose a main variety of Spanish among the ones that were used in the kingdom of Castilla y Leon.
So I doubt any previous change in the language was a coincidence or fashion: the population that spoke old Spanish was so big and diverse that it’s nearly impossible for them to suffer the same events.