have you got any ornaments in the tank that are a bit on the rough side? Also how often do you vac the sand?
All I can think of is they're rubbing the barbels away on something else that is rough or that they're are getting worn away from dirty substrate (which I doubt, but it's all I can think of )
its been a week since i cleaned the sand out but i have wood in my tank which now that i think about it they do get a little crazy with the cleaning and will clean my wood from top to bottom and even under neath even digging up my plants