Earlier quoted context omitted.
I'm hardly qualified to comment on any specifics. I'll only go as far as saying that Grass wasn't well-known for his love of snack food, and I interpret the poem as referring to his being argumentative, even beyond his lifetime. Maybe someone else can chime in.
There's a phrase in German language, "eine harte Nuss zu knacken haben", literally "to have a hard nut to crack". Maybe that means that he'll be cracking nuts (problems) even after he's buried?
"Wenn es dann kracht,"
you can also translate as
"where there's a fight"
so there's a bit of a double-meaning here - "krachen" means that there's noise from his crushing nuts, but it can also mean that there's a conflict [1]
[1] http://www.duden.de/rechtschreibung/krachen "(umgangssprachlich für streiten)"