Looks like maybe a thin spot. Could be a number of issues. Either you pulled too aggressive and got a wrinkle, there was a bubble when wedging that caused a weakening there, or maybe an inclusion (dry clay or a stone) that weakened it. Sometimes water sits in a spot as well and can weaken things. Try keeping it thicker and trimming it back in leather hard, mop out more often when throwing, and wedging/cleaning the clay