Branching Out
Deeply Rooted
I have always associated the leaves hanging down in winter with a lack of moisture. This article does a good job of explaining how difficult it can be for rhodos to stay hydrated when the ground in frozen solid:
gardenmentors.com

Why Rhododendron Leaves Roll Up in Cold Weather
Noticed your rhododendron leaves roll up when it's really cold in winter? Learn why curled rhodie leaves may be how plants protect themselves.