It was fresh... literally just collected it from the coop that day and cleared out the eggs the previous day. It was pretty thin feeling though...Were the shell pieces a normal thickness?
Strange shapes can cause the shell to be weaker just because of the shape.
Do you know for sure it was a fresh egg? Eggs that got hidden for a while, and started to rot, are much more likely to explode. Even a very strong shell can explode if the inside is rotten.
Other than that, just letting the chickens have free-choice oyster shell usually works fine. A few rare chickens either won't eat the oyster shell or have some problem where they lay weak eggs anyway.