We had a landscaper come in, pull up some bushes and plant 3 arborvitae trees in its place. One of the trees died this year. We replaced it (in the exact same spot) and now it is dead, too. The other trees are all fine, which makes this puzzling. The alkalinity is around 7 for all three trees, unless make alkalinity measure is broken.

The pH of 7 should be fine for Arborvitae and is not likely part of this situation. While all were the same species, each plant develops its own root system. The first one that failed may not have had a sufficient root system to survive the transplanting into your yard. Larger (older too) plants have a more difficult time establishing than smaller and younger plants. If you are going to try a third time, dig up the dead one carefully and examine the roots for signs of recovery. If there was not indication of callousing or new roots, the shock of transplanting was likely the cause Disturbed soils can hold more soil moisture and drain poorly untill they settle and return to their former condition. If there were a lot of wood chips created in the removal of the old plants, that can also influence soil structure.