2000 was the first year for the Sac dollar, and the earliest coins out of production tended to turn a puke greenish brown. It was thought that this was due to an improper rinse which the Mint finally corrected. Chris
While there are the improperly rinse dollars, there are also improperly annealed. Neither of these are the case. This has likely been buried in the ground or something similar causing the metals to oxidize. Environmental damage.
It's very possible. I only mentioned the improper rinse because it became a topic of discussion in the trade magazines in 2000. Perhaps @paddyman98 has some photos of Sac dollars that he found while metal detecting. Chris