Never heard of it before in sports.
I hadn't either, but I just assumed it was because it wasn't publicized, which is kind of the point of an NDA anyway, which is discretion against dirty laundry and trade secrets.
I don't know how it applies to the NFL, because it's a monopoly, but also because your competition (other teams) also share in your profits and success (the NFL as a whole). It would kinda be like signing an NDA at a company and then getting hired at a different sector of the same company but not being able to discuss your previous employment.
I ultimately don't know at all, but I don't find anything scandalous at the thought of it, at first glance.