Derivation of the Macdonald operator $D_{n}(X;q,t)$

Question

Since I first encountered Equation (3.2) on p.315 of Macdonald's Symmetric functions and Hall polynomials, I have wanted to know where it comes from. So how does one derive the operator

\begin{equation} D_{n}(X;q,t) = a_{\delta}(x)^{-1}\sum_{w \in S_{n}} \varepsilon(w)x^{w\delta}\prod_{i=1}^{n}(1+Xt^{(w\delta)_{i}}T_{q,x_{i}}) \end{equation}

in the first place?