I think someone wrote above that the pig could be referencing cops, and then I read the third stanza again and now I concur.
I see a parallel between the oppressed and the oppressors in this poem to that relationship between whites and blacks in the antebellum south.
I think this is a stretch, so I will explain. There is a constant motif of generational turmoil, "mothers hardening like pounded stumps," and "from my children inherit," or "out of buried aunties". Which makes me think of the generations that were born and died within slavery.
There is a strong emphasis also on religion. As someone pointed out earlier, "They Lion grow" could be a reference to Jesus. Phrases like,"pig driven to holiness," and , "all my white sins forgiven" to show the emergence of christianity within the slave population.
Lastly, there is a deep connection to the earth and nature throughout the poem. If the lion is a reference to Jesus, it should be noted that he is represented by an animal, the oppressors are pigs and hams respectively. I get the sense that nature is a driving force and on some level, worshipped, in it's own right by africans who live off the land.
Like I said, a stretch. To wrap it up, I would say that the poem creates the idea that we are slaves to waste and pollution.