A few possibilities come to mind. "Digger" is an antiquated derogatory term used for Native Americans in California and some other regions of the West. Also, some people are using it as an Ersatz for the N word in an attempt to veil their racism or bypass automated filters, much like how my inlaws think they are so clever by referring to non-whites as "Canadians." Racists are rarely as smart as they think they are.