Do you think it was a casing / topping they added, or somehow naturally occurring
Some of it was fermentation. Some of it was acetic acid used as a mold inhibitor. The vinegar generally evaporated off when smoked. Or you could open a tin a week or two prior to smoking, and vent it for a minute a day to remove the vinegar and reveal the fine Virginia underneath.
I didn't jump on the McClelland bandwagon until fairly late. I liked some of their Grand Orientals and their red Virginias. So I don't have a lot of McClelland cellared, around 30 lbs, but that will do just fine. There are a few of their bulks that I wish I had stocked, or stocked more, but I'm not hurting.
So many other great blends to smoke.