I’d wait for more study before taking this too seriously. As Chris Chatham also notes...
If stepping backwards is associated with the spreading of spatial attention (as seems reasonable—to make sure you don’t fall over!) or simply with less acute vision than stepping to the side or forwards (as also seems reasonable—vision is probably better for objects that approach or stay the same than for those that at least momentarily recede), these effects could be just another demonstration of the well-known sensitivity of the Stroop task to vision and spatial attention.
I’d wait for more study before taking this too seriously. As Chris Chatham also notes...