The theory is that when you go carnivore (or any other way of eating that's pretty much zero oxalate), your body starts dumping oxalic acid. Can range from uncomfortable (I had some weird kidney aches) to serious (kidney stones, eye issues, rashes). One way to mitigate is to keep a small amount of oxalate in your diet, presumably so that your body works out the old stuff but not all at once? Black tea has 40mg or so of oxalates.