Cracked tiles in shower?

I built a ceramic tile shower about 6 months ago. I was surprised to see that some of the tiles cracked on the inside of the shower on the vertical piece where the shower door is. Underneath is solid, 4x4 covered with 1/2" Wonderboard strips. The tiles are mounted in thin set mortar. The whole house is on a concrete slab, so I don't believe there is any movement. Could it be thermal effects due to hot water use? Thanks.
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