If the concrete is slightly higher (a fraction of an inch) there will be more of the cutting edge contacting the surface, therefore more wear in that location.I am with Hydro on this one. It is concentrated on the edge where the joint is. I have a feeling there is steel edging or some sort of steel like rebar as others have suggested down in there. When the snow melted it puddled and came up to the top of the cement and had rust ore in the standing water. Also what chemicals if any were used on this drive.