Is your OAL longer than the factory ammo? Is the bullet hitting or close to the rifling?
I looked on the Berger bullet site and their load data lists a MAX charge for their 300gr bullet with Retumbo at 89.5gr. Berger lists a COAL of 3.681" for those loads. According to the article I read in the Aug 2014 Handloader the 338 Lapua is susceptible to high pressures even when within the limits of the load data. Your charge being slightly over the limits plus the other variables might have pushed the pressures too much.
338 Lapua Magnum | Berger BulletsBerger Bullets
The Handloader article is a good one, August 2014, #291, page 46.