Even if NGA had a completely un-sullied reputation, that ammo must be going on 2-3 decades old.

I have heard/seen lots of discussion on whether or not powder gets old and have seen some "evidence" that some powders can become unpredictable (in terms of burn rate) with age and probably incorrect storage.
My understanding is that the burn-rate-modification layer on double-based (usually ball-type) propellants can break down.

Typically when this happens it will also leach some of the alloys out of the case, making visible marks on the outside.

I have however shot quite a lot of very old ammo myself, and apart from misfired and hang-fires never had any issues.