Other trapped graves in the game usually explode as an invisible mine at the center of the grave. You can usually avoid death by digging with a character who has high CON and fire resistance while the rest of the party keeps their distance - that's how I dug up every grave around Cyseal's old church without having to reload.
I am NOT going to ask how you figured that you have to use a character with high fire resistance. Instead of air, water, earth or poison resistance. Quite frankly - either pure luck or the first bad experience and a re-try I guess?
Regards,
Thorsten
