When does this happen?

Consider a parabola $y^{2}=4ax$ with $a>0$ . Points $A, B, C$ with coordinates $( x_{i}, y_{i} )$ , where $i={1,2,3}$ , respectively, on the parabola are such that orthocenter of the triangle $ABC$ is the focus of the parabola.

Find the value of $\left( \dfrac{y_{1}}{2a} -1\right) \left( \dfrac{y_{2}}{2a} -1\right)\left( \dfrac{y_{3}}{2a} -1\right)$ .