Why are powder XRD axes sometimes plotted in reverse?
Best reason I can think of is that you're being given data in momentum space rather than position space.

You can convert from "position-time" space, which is what we're comfortable with as humans, and describe the system in "momentum-energy" space, which is simpler from a physics point of view. Position is the reciprocal of momentum and energy is the reciprocal of time. If you convert the diffraction equations to momentum ($k$), which is proportional to $\frac{1}{sin(\theta)}$, then the diffraction equation $\lambda = 2dsin(\theta)$ is transposed to $k' = k + K$ where $K$ describes the peak in momentum space, $k$ describes the momentum of the incoming photon, and $k'$ describes the diffracted photon. This equation is linear and much easier to manipulate than the "real-space" form of $\lambda = 2dsin(\theta)$.

The equation in "momentum" space (k-space) is difficult to understand but is nice and linear from a physics standpoint.

https://en.wikipedia.org/wiki/Crystal_momentum