Is it short of water? If not, then look for signs of infestation. If none, then I'm sorry, I think it may well be on its way out.

If it's just the inner parts, I wouldn't worry. Many types of conifers naturally only have the fresh good-looking foliage on the outside/ends of branches. The inner areas just turn brown as the plant grows. Yours may be absolutely fine.

As Longleaf says, probably the plant naturally shedding what it doesn't need. The inner leaves are starved of light, so fade out and fall off. In dry conditions this natural process is occasionally exaggerated. I wouldn't worry yet. Phil J