I've done some searching and can't find a reliable source for the origin and exact meaning of this term, but there are a few options. The term is not used in any specific context in the song.
A fellow African American told me that it is a racial statement against white people. "Cracker" is often used by African Americans against white people. Much like the "N" word is to them. So, if the party is "Crack-a-Lackin", it means there are no "crackers", i.e. white people, at the party.
I shake my head in disbelief when I hear this term being spoke by kids and in movies while African Americans laugh about it because white people are too stupid to understand it is a racial slur against them.