Provoke does not work on hunts.

What was the item level of the DRK and what was the item level of the AST? If the DRK was item level 560 and the AST was item level 660 then it's possible it's the item level difference. Tanks are pretty good at holding aggro even while far below the item level, but the higher the difference in item level the more possible it becomes.

As the other posters said, A ranks are designed for a party of 8, although they do become quicker to kill with smaller parties as item levels increase.