Happened to me too. I was going around just knocking out Raids, and then when I came back from doing one I looked at the map and all of the sudden there's an entire new map of Raids.
I looked around on Google and most sites I found were about intentionally pushing the clock forward, but I did eventually find
a GameFAQs thread with people having a similar issue (and no definitive answers). Someone notes this refresh seems to happen every 5 days, though that was just an observation of three happenings of it and no other posts following that thought were made. It's also noted(?)/suggested that clearing the story and going online may cause the resets, though that doesn't seem to factor in on this particular issue.
BTW, this random reset also affects Mass Outbreaks, may possibly also affects Auctions too.
Right now it seems like a mystery, one that no one really seems to care about has it happens too infrequently to matter.