I think if you're working with a completely alien ecosystem, you can get away with whatever you want, and, given we only have the single reference point, it's almost not worth speculating.
So I'll shift the focus a bit to the nearest situation I can think of on Earth: birds. Flighted birds use their forelimbs for locomotion (in a sense...) and their rear limbs (in some cases as least) for fine object manipulation.
So, in terms of ending up with something like a Star Wars Dug with a contemporary terrestrial starting point, I can imagine a situation where a bird population become isolated on an island habitat with no native predators, and, in common with a lot of real bird species in this situation, undergo insular gigantism and lose their capacity for flight. However, unlike a lot of existing flightless birds, whose wings tend to atrophy away over time, becoming more fully bipedal, this bird species begins to re-evolve a form of quadrupedal locomotion.
At first this is very clumsy, perhaps the birds shift some weight onto their "elbow" joints to allow them to keep a low profile, to sneak up on prey in tall grass, but over many generations, the wing starts become something much more like a leg. However, while birds typically have quite pronounced digits on their back limbs, the anatomy of the wing has the fourth and fifth digits completely atrophied away, and the first, second and third fused into a single wing bone. As a general rule of thumb, evolution seems to find it a lot easier to lose or fuse digits than it does to regain or de-fuse them, so the "foot" at the end of the forelimb is likely going to stay a much more simple structure than the foot on the back limbs. Perhaps the feathers will adapt or fuse in some way to create something along the lines of a horse-like single-toed hoof.
Fast forward a few million years, and a change of environment has pushed this lineage into a more arboreal setting. The front hoof has evolved into a hook for ease of climbing, but when this creature needs to use fine motor movements to peel a banana or deshell a nut it uses the bird-like talons on the end of its back legs.
Let's then say, further in the future, this little bird-monkey faces the same evolutionary pressures our own ancestors did, and is forced out of the trees and onto the open plain, and the same incentives for upright bipedalism apply. At this point, the wing-leg-hook-things begin to lengthen and strengthen, and the creature adopts a posture at first somewhat similar to great ape knuckle-walking, and then may eventually begin to keep its whole back half of its body lifted off the floor altogether, and carry its weight just on its "arms", and there you have it.
But, to go back to a completely alien environment, I think my only advice would be to acknowledge that the forelimbs really are the obvious default choice for throwing, carrying, manipulating, and all these sorts of fine movements, because it's easier to balance the weight on the back legs, the front limbs are closer to the face so easier to visually co-ordinate with, and so on, and so if you want to swap it around, make sure to have a good historical reason why the obvious benefits of the forelimbs won't apply to your alien.
