As an aerospace engineering student, let me hazard an (educated) guess. I've never in my life seen a forward swept fin, so I'll try to relate them to forward swept wings which I know a bit about.

• Before the air reaches the wing, it is traveling parallel to the body. Once it hits the wing, it experiences a spanwise flow. For example, if it's an aft swept wing (normal), the air is traveling parallel to the body, hits the wing, and then leaves the trailing edge (again parallel to the fuselage, but farther out on the wing, further from the centerline of the aircraft). Does that make sense? It's hard to explain without drawing a picture. In a forward swept wing (fsw) the air will hit the tip of the wing first, and leave the trailing edge closer to the fuselage than it started. Basically the flow in asw starts at the root and ends at the tip, and the opposite for fsw. This means that if you have control surfaces (which are usually near the tip), you wont stall as easily in a fsw configuration, allowing you to fly at higher angles of attack. I see no control surfaces on this missile, so this is irrelevant.
• fsw will generate more highly swept shock waves (i don't think a model rocket will go fast enough to generate shocks (correct me if i'm wrong) so this, too, is irrelevant).
• Will possibly have reduced subsonic drag (potentially a reason for doing it this way, although the added benefit for fins, which are much smaller than wings, will be negligible).
• FSW have serious structural problems, or at least used to, which is why you don't really see them at all on aircraft. Again, this rocket is probably not going to encounter these problems at the speeds it will experience.
• The center of pressure is closer to the fuselage for a fsw, meaning lower root bending moments. But then again it should be a symmetric fin so it won't apply.
• There are several other consequences of using fsw instead of asw, but as they apply to things like radar cross sections, lift, and control surfaces, etc (none of which are applicable here) I won't go into them.

So basically (once again let me say this is a guess) none of the benefits OR the detriments of a fsw seem like they would apply in this situation. I believe that they are inverted only because someone thought it would look cooler that way. This conclusion is supported by the fact that no real missiles have this setup, which suggests another possibility: that it could be this way precisely because it is a model. (Eg: easier to manufacture/put together/whatever)

Edit: It's also possible someone made a mistake and just went with it.