This is due to its inability to withstand thermal gradients across its width. Typical sintered alumina is refractory, but it is not thermal shock resistant like tabular alumina. The inner part of the shelf was being protected from the rising heat because of this heavy, slow-to-rise calcimine vessel on top of it. The moment of the crack was so dramatic that, in spite of the weight on top of it, the shelf blew apart leaving 4 pieces with an inch-gap separating them.
When sudden changes in temperature cause dimensional changes ceramics often fail because of their brittle nature. Yet some ceramics are highly resistant.