I think the idea of a combine for out of work FAs has been around away, but never got more push other than a former players suggesting, agent or media type one liners about how they would like to some pro day or combine such event to show they can still play. I think Terrell Owens was one that has talked about such thing, to show he can still play.
Why now? ... I think because of Matt Birk, as he seems to be running the thing. Birk has been NFL director of football development for less than a year now, announced on July 10, 2014. And Birk has talked about a lot of changes up... including doing studies for the combine to see what shows the players skills the best, and possibility changing the combine events... or even making them more personalized for the position. Such as not making the linemen run the 40 yard dash... instead making them do only a 20 or 10 yard dash. And coming up with new stuff too.
I think Birk claimed they took applications for who wanted to be in the vet combine, and then they picked who they thought would be the top 100 or so. So those there weren't there, either didn't put in the application (which cost money... I think PFT.com said $400) or they weren't thought to be in the top 100 or so by the NFL people.
Though the Cardinals were the first to sign someone that was at the vet combine, they signed WR Nathan Slaughter. Of course coming out of college Slaughter was SMALLER than 5'9", 185lbs ... and he's a speed demon and the Cards head coach loves small speed demons for some reason.