I'll preface this with this is 100% pure speculation and it's based on hearsay but mysteries like this very much intrigue me so I'm going to give it a shot.
Things we've seen:
1. Carney is on a wildly short leash
2. His first appearance was at Colgate when Rickie's helmet came off (not when Rickie got pulled for throwing an FCS Int, the starter actually came back in), and on that one play he immediately scored a TD and then disappeared.
3. The team confirmed the redshirt.
4. He's been kept out of obvious throwing situations and his redshirt was burned in just about the most useless way possible.
5. Luke said he would've re-evaluated the redshirt if he got the UNC start, but literally the walk-on got it over him.
The limited rumors we've heard:
6. Luke Carney's dad is somehow connected.
7. The team has suffered parent involvement issues that started around Game 3.
Game 3 was, notably, Colgate. If I'm Carney Sr and watched Rickie's rather emetic performance against Colgate, along with the staff's apparent disregard for Carney as an option, I'd be pissed. At the same time, if I'm Nixon, and if a dad started messing with my work? At that point I'd do well to keep the situation at arm's length.
Again, I'm jumping to conclusions here, but it's too curious a situation for me to not think about it.