It has to be the altitude. I would guess there are pockets of air and water trapped in the shell that expand at the altitude in the mountains. I live on the Front Range at about 5000 feet, and oftentimes when I buy some product like yogurt at the grocery store that is packaged with an airtight seal, it is inflated and distended because of the lower atmospheric pressure here compared to wherever it shipped from. I've never had anything literally explode, though. In the mountains at 10,000 feet or so, that effect would be even more pronounced.
The shells from Bula are probably coated with some kind of sealant, which could be keeping the air in. You could try drilling some small holes in the outside of the shell to let the air out, maybe. (That is holes in the outside that do not go all the way through, obviously)