Life becomes difficult for Mule Deer when snow is deeper than than 30 centimetres. As deer must stay on the move both to find food and escape predators, deep snow is problematic.
What these pictures from a few weeks ago show is that if you are a Mule Deer and the snow is deep, optimal strategy is stick together and follow the leader.
The Kootenay–Lake Website offers a discussion and more pictures of local Mule Deer.
Two recent views of Mule deer courtesy of Doug Thorburn.