There are 2 schools of thought on what causes this to happen. There’s the “a bubble was left” theory. And the “it wasn’t really dry completely through” theory.
I’ve seen some of the more educated potters subscribe to the “it wasn’t completely dry” theory. And, for thick solid pieces like this, that makes a lot of sense to me.
You might make it more hollow. Or – take it home and put it in the oven around 200F for a day or so.
I saved the major pieces, but there were so many slivers that went in the trash that you probably won’t be able to Bisque Fix it back together.