Didn't they say that they came from the Poketransfer breaking open or something? I could be wrong but that was what I remembered.
I do think that we're plopped into Unova within a few months of the event happening that causes it though. The way I see it, we're in a frozen moment in time, where a few Pokemon blossomed into a lot in half the region. If we had been there a year before, there would have only been Unova Pokemon. And as we saw, 2 years later and the entire region is overrun with past gen Pokemon.