Thuja is a funny tree in that, more than any other tree I can think of, branches depend on certain roots. Old trees in the wild will sometimes have almost the whole tree rotted away except for one or two life-lines (veins to some) that feed a branch each. For that reason I would suspect mechanical root damage, or possibly salt if near roadway, sidewalk, or driveway. Thuja isn't too susceptible to winter burn, so it's not likely the cause, especially if only individual branches are involved, but it's possible if the damage is on the south to southwest facing portion of the tree(s). It's so difficult to guess w/o seeing the damage.

Al