It's quite common to see turboprops having a wing with the outer part of the wing with a small sweep (around 3 deg at 1/4 of the chord). What's the reason? It's usually stated that wing taper is used mainly for reducing the critical mach number or to fix the CG problem. For turboprops such as ATR-42/72 or Dash-8 the effect of Mach number is still negligible and it's unlikely that both of these planes and their subvariants have this feature because they needed to fix the CG.
Don't think of those wings as swept. As far as aerodynamics are concerned, they are straight wings.
Think of them as tapered -- The taper is there to improve the lift distribution.
The line about which the wing has zero sweep is chosen for non-aerodynamic purposes (since there is little aerodynamic benefit or penalty to one choice or another).
In these cases, the wing is un-swept at the mid-chord or possibly the 3c/4 line. This is usually for simplicity of layout of structures and systems.
In a related manner, some aircraft twist their wings about the line that the control surfaces are attached to. This leads to a straight and true aft spar that is easy to hang control surfaces from.
It's quite common to see turboprops having a wing with the outer part of the wing with a small sweep
At least for the ATR-42/72/Dash-8, the wing possesses no sweep as it can be gathered from the following cutaway (source):
As it can be seen, the wing spars (185 in that picture) are basically straight and perpendicular to the fuselage. An angle different from 90° would denote a swept wing, which is not the case here.
What you're referring to is actually the taper ratio i.e. the fact that the chord length reduces spanwise from root to tip.
What's the reason?
This is done to reduce lift (which is proportional to the chord length) from root to tip in order to optimise its spanwise distribution, reduce the relevant induced drag and optimise the wing structure, as explained in several answers here, like for example this one.
It's usually stated that wing taper is used mainly for reducing the critical mach number or to fix the CG problem
Tapering the wing doesn't really help in reducing the critical Mach number: sweeping the wing and choosing an appropriate airfoil (so called supercritical airfoil) reduces transonic effect with the relevant drag increase. If you find a CG problem after the design freeze then changing the taper and/or the sweep angle is definitely not a quick way to fix this problem since it entails a major (and costly) redesign of the wing itself.