Its actually not that close to the A76, the road is a busy road, but i think it might be far enough away not to be the cause. The walls themselves are enclosed by thick bushes, trees etc, again giving enough protection from exhaust pollution. I still think the main reason may well be the exposure since being uncovered, no natural protection, aswell as other pollution and man made effects.
Sandstone is a really soft stone, normally the carvings would have weathered off years ago, but something was keeping them safe from harm. Now after being exposed again, the weathering seems to have speeded up very rapidly. Maybe the rock surface had a source of protection, maybe moss or something similar, the process of cleaning the rocks by Morris helped to start off the process.