Okay, I found this thread after my own bottle of French's Worcestershire sauce not only popped and produced a cloud of fog, but actually foamed up like a beer!
I called the 800 number on the bottle. The rep I spoke to said this is actually normal for fresh bottles. I don't remember seeing it happen before, but since I tend to go through a bottle about once every ten years, that's not too surprising. It's also not surprising if you remember that Worcestershire is a fermented product.
If it does
kill me, I'll be sure to post a followup message warning everybody.