Why are the load resistance and reactance in parallel?
They are in parallel because they were obtained using a parallel model.
The author first estimated the load \$S_{pu} = P_{pu}+jQ_{pu}\$.
Consider \$Z_{pu}\$ to be the load consuming \$S_{pu}\$. This load can be in series:
$$Z_{pu,Series} = R_{pu,Series} + j.X_{pu,Series}$$
or it can be in parallel:
$$Y_{pu,Parallel} = \frac{1}{R_{pu,Parallel}} + \frac{1}{j.X_{pu,Parallel}}$$
You see, both approaches are possible, and the results should be equal, but the values of resistantes and reactances will not be equal, due to their modeling.
Thre problem with the series approach is that we need to estimate the current so that we can calculate \$R_{pu,Series}\$ and \$X_{pu,Series}\$. Since he has the power injections \$S_{pu} = P_{pu}+jQ_{pu}\$ and the terminal voltage \$E_{pu}\$, obtaning the values of the parallel approach are quite simple:
$$
R_{pu,Parallel}  = \frac{E_{pu}^2}{P_{pu}} \\
X_{pu,Parallel}  = \frac{E_{pu}^2}{Q_{pu}}
$$
That's why he modeled the load in parallel.
P.S.:
As a side note, to obtain the model in series, we first need to calculate the current:
$$I_{pu} = \frac{S_{pu}^\star}{E_{pu}^\star}$$
Then, Z can be obtained as:
$$Z_{pu,Series} = \frac{E_{pu}}{I_{pu}}$$
Although this is a phasor approach, and the computations will be more complex.