Transplanted my giant sequoia seedlings

I transplanted four giant sequoia seedlings from cell-packs into 8" pots today.  Last year I did the same thing and they all died.  I'm not sure what happened, but they turned brown from the top down, one after another, during a time that it was very hot in San Jose but I was frequently misting and watering them.  My list of possibilities includes over-watering, too much sun, or botrytis.

This year I transplanted them to soil amended with both sand and gravel, for improved drainage.  I'm doing this earlier in the year, in San Francisco rather than San Jose, so they shouldn't get burned.  But I can't do anything about the botrytis, if that was the cause of death last year.

Greyneedle says that I should expect only 25% survival.  I'm not sure if this is true all over the country, or only on the east coast.  Why should 75% die here in CA, under optimum conditions?