I think when the game started out there was a lot to learn so there was more involvement, now at least in our league, the players know what is going on and unless there is something out of the normal you do not get much input. I noticed the decline last season and this season it has grown. Also of course you have more outside interest right now between the Olympics, baseball getting ready for playoffs, football starting and of course college both games and school starting, and Madden just came out too.
I would expect early to mid September for the activity to start back up again. Except for the sad folks who are on here all the time

And yeah, the team owners and GMs are the ones who for the most part stir the pot in forums. And yeah, once players get their build like they want there really is not much for them to do or be a part of. And it is a damned if you do, damned if you don't for the amount of players per owner. You can try to build a team of one player per owner but you then run the risk of inactive or 'My friend has a team now I want to join them' players and you can lose 1/2-1/4 of your team this way a season. With nothing you can do to make players want to stay there is not much an owner can do to stop this. So you are almost forced to go for solid owners with multiple players to keep a team active and together which then of course is less input so lower forum activity... and round and round it goes.