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.