[ysavir]

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.

[elnatro]

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.