It's very simple. Put a winning team on the field and these tickets are not available to the opposing teams. During the Payton/Brees era is was rare to see a minimal amount of away teams represented. That is not the case the last 2-3 years.
Eh, there have always been visiting fans. Like fairly significant numbers. It's nothing new. The visiting fan numbers may be up marginally, but not really that much more except for maybe a fee games here and there.