This is stress cracking from brittle plastic plus temperature swings and oils. The cracks start at thin spots and cutouts then spread with every flex or drop.
Go for a flexible elastomer with a thicker bumper around the corners and no sharp edges near openings and it will absorb impacts instead of splitting. Wipe grit and chemicals off at the end of the day and do not keep snapping the case on and off, and avoid leaving it baking in the sun or freezing in the cold. Not a defect. Physics.
