Weak acid titrated with strong base: Why's the 2nd derivative of the curve negative after the equivalence pt?

The direct curve plots pH against volume of base. The slope of this is initially near zero, rises to a maximum at the end point and goes back to near zero after the end point. The first derivative curve plots the slope of the direct curve so this will appear as a peak, the graph having a positive slope before the peak apex and a negative slope after it. The second derivative plots the slope of the first derivative, which will be positive before the end point, zero at the end point (the apex of the peak on the 1st derivative plot) and negative thereafter