Jon - the reason the trail were damaged by the rain is that the walkers, horses and bicycles create the conditions for soil runnoff
In area with a lot of peat or with fragile mountain siols erosion leaves huge scars. Maidens in the pentlands used to be a 30+ ft wide bog before it was armoured, Ben lomond path is a scar that can be seen for miles, same with other mountains.
In the time I have been going into the mountains both by foot and bike ( 40 yrs) I have seen huge erosion and while other countryside users cause much I believe that mountainbikers are fairly unthinking or oblivious to the damage they cause - especially when some damage could be easily avoided

