If I knew why, I could fix them but the manufacturers won't release any service information on things like that. We can't even buy car radio service manuals anymore because they want to keep all that lucrative repair business for themselves.
If you do an internet search for GM digital dash repair, you'll see there's a lot of places that specialize in repairing them. It's a real common problem and is why I stick to normal gauges. I used Advanced Auto Electronics recently and had a very good experience with them.