No expert but I'll guess. I can only see it being one of two things assuming your ammo is good.
1. Maybe it wasn't shooting as good as you thought at first. Did you shoot enough to confirm it was shooting good, or did you possibly luck a group together.
2. Your Lead Sled combined with a light weight 338WM has destroyed yet another scope. Lead Sleds have been known to destroy scopes fast with heavy kickers when enough weight is used to immobilize the rifle.
Now those suggestions are based on good ammo and assuming your barrel isn't slicked over with copper.
