I was out collecting samples and took a tumble on a shale layer. It broke apart like dry crackers under my weight. My friends thought it was hilarious, but it made me question things. I honestly think shale is weaker than most geologists admit. They call it a solid rock, but my experience says otherwise.
My laminate floor chipped from a dropped mug just like your shale cracked. It's the same story with stuff that looks solid but has weak layers. We build on shaky ground all the time and act surprised when it fails.
Remember my buddy who tried to build a fire pit with shale? Looked perfect until the first real heat. Whole thing just split apart like a stack of plates.