Thanks JZP.
So from your post it appears you had overstretched, weak Upper Traps and probably LT was already strong to begin with. By strengthening the Upper Traps, stress is being taken off the clavicle by the muscles below.
In my case (I think), I am already Upper Trap dominant (a very common phenomenon) so Upward Shrugs should be added only after I manage to make my UT:LT ratio dysfunction less. Otherwise the strong Upper Traps will overpower the LowerTraps again.