It was rain that caused the R&BT slips. It always is in that area. West of Scotland gets a lot of heavy rain and always has had. The old military road, as it is called (I think Redcoats may have made it; they certainly made a bridge further down the valley), was opened and a convoy traffic system set up. The old road is lower down the slope (steep valley side) so even if affected it might not have benn as bad as the high road.
You take the hogh road and I'll take the low road, etc...
Long ago this area was under ice too so I suppose you can blame climate change for this too if you want to only that loss of ice was nothing to do with anything human beings did, it was just down to ...er... the fact that climate changes.