It's hard to say why. Arborvitae like a lot of water, but you say you watered well. Usually you don't need to fertilize newly planted shrubs, and if you do, it should be very diluted. Besides wilting, are the branches changing color or drying up? Are the roots sticking up from the ground? Sometimes when you water, the dirt washes away from the trunk and exposes the roots. Arborvitae are very sensitive to that.