1. Pressure from suboptimal (lower back) posture where the back is slouched too much - I don't think I have this problem but it's possible I do this unconsciously when my legs get tired at the end of a hard run, etc.
2. Shocks from unexpected bumps - when I hit an unexpected bump or incline, it goes right to my lower back. **Choice of runs - obviously I start my day early on smooth groomer runs, but there were a few busy days during past two weeks when the runs got choppy really fast.
3. Wear and tear from excessive skiing - I don't think 4 hrs every other day for two weeks is anywhere close to being excessive, but coming from no workout/exercise for many months, it could be, and I do push hard when I ski. Past two weeks, I have been working on getting my skis far out as I could on quick SL turns.