I wonder if the increasing percentage of digital sales has more to do with and overall increase in the percentage of games being purchased digitally, or some other factor.
Something else. It just shows, at least for this game (though I'm sure it's similar for most games), physical sales greatly outnumber digital (which it seems the average for big games is ~28%-30%) in the beginning when there is price parity. However, after a year or two, that's when you start seeing killer deals on Steam and PS4/XBO. It'll be marked down to $5-$10 in a flash sale, while those prices are reserved for BF at retail.