Magnet,
I haven't found this to be a problem for myself.
Are you applying a haul while casting? A short, quick/hard haul will break down a fly line's coating significantly faster than a longer, smoother haul to provide that extra "umph" in turning over the long leaders. The quick exertion of power during a short, quick/hard haul will make the rod's guides dig into the line's coating more than usual.
Other than that, I would assume it is the coating on the line? I've been using the Cortland Western Drifter and Platinum lines over the last 4 years and haven't found them to wear out quickly by any means.
Mike