> Apathetic non-voters
An important thing to keep in mind in American politics is the massive amount of voter suppression. Not voting doesn't inherently mean you were lazy or apathetic. It may well mean your vote was suppressed by any of a hundred tactics. Closing polling places in blue regions, requiring in-person voting on-the-day, restricting early voting, restricting vote by mail, failing at sending people ballots, spuriously dropping voter registrations...
> requiring in-person voting on-the-day
Exactly three states don't offer early voting to all voters [1] and none of those three were battleground states.
[1] https://www.cbsnews.com/news/map-early-voting-mail-ballot-st...
It would be a tall feat to suppress close to a third the population from voting!
All that is true, and to a great degree the reason why the concept of "swing states" (or rather the "non-swing states") even exists.
It does not explain however why almost all the swing states aligned with Trump this time.