CalCat --
Brett (Treeman14) has good info for you in his post.
In my experience, I'd rank the problems:
1. Nursery stock - poor root development, check for pot-bound, girdling roots, circling roots, and/or trunk scrapes.
2. Planting - inspect for mishandling (trunk scrape, especially near root flare) and/or buried root flare (should usually be 1- to 3-inches higher than surrounding grade).
3. Watering - too little (maybe too much, but less likely) - especially to existing rootball.