Exploding Ink Bottle

So I recently opened up the wooden box I keep my Winsor & Newton drawing inks in, and was surprised to see that the bottle of Silver Ink had exploded. The glass bottle was still in one piece, but the plastic cap was in several pieces, and the cardboard box had blown open from the inside, and silver ink was all over the place. I posed this on the forums of wet canvas.com, and asked if anyone had this happen too. The responses ranged from amazement to skepticism, but obviously no-one there had ever had this happen to them. I'm still at a loss to explain this.