It's a combination of factors, but the relatively new focus by an increasing share of the American public on getting outside, getting exercise, etc., is certainly one of them. I'm sure the NFL has survey data and they use that to inform initiatives like NFL 60 (getting kids outside to play), just as they presumably use data to create programs honoring the military and wearing pink shoes. The NFL undertakes these initiatives in response to business and PR needs, and not because it is benevolent, so it is real.
Again, it's a combination of factors, and it's hard to separate them from one another without raw data. Another could be the 18-34 demographic's view of the role of parents, which contrasts to the older demographic's more traditional roles (i.e. men are more involved with kids and don't head to the bar for two games on Sunday). That demographic is where the NFL is seeing the biggest decline. Another factor that could be impacting viewership is the decline of the New York teams, with their notoriously fickle fans. So if you're a 32 year old in NYC, in a survey you might say you would rather go upstate to pick apples with your wife and friends for the day than you would be to watch the Giants.
Again, it's a combination of factors, and it's hard to separate them from one another without raw data. Another could be the 18-34 demographic's view of the role of parents, which contrasts to the older demographic's more traditional roles (i.e. men are more involved with kids and don't head to the bar for two games on Sunday). That demographic is where the NFL is seeing the biggest decline. Another factor that could be impacting viewership is the decline of the New York teams, with their notoriously fickle fans. So if you're a 32 year old in NYC, in a survey you might say you would rather go upstate to pick apples with your wife and friends for the day than you would be to watch the Giants.