Well, it's definitely something to keep in mind for later on.

[ There are so many things it could have been. Construction obstructing their usual migration path which might force them to change it slightly before heading back. A drought or any kind of natural disaster that might have scared them off. Heck, even these regions just might not have what they need in order to sustain their lifestyle so they might have returned out of desperation. ]

Animals and Pokemon are too strange to understand at times. Just tuck it away and if you see something relevant later on, it might be related.