Answer: Based on the information in your description it is impossible to determine why the plants died. However, here are some possibilities. Newly planted plants require adequate moisture until they become established, and shade plants in particular tend to prefer a moist soil. They could have died as a result of the soil being allowed to dry out. If the soil was improperly or insufficiently prepared, the new roots would have had trouble penetrating the soil; again, this could cause them to die. If the planting location is sunnier than you realized, with say afternoon sun reaching the plants, then it may have been too sunny for them. This can kill shade loving plants, too, although it may take until about mid-June for the heat and sun to become intense enough to do them in. Overfertilizing can "burn" plants and in extreme cases cause them to die. Finally, accidental exposure to herbicide can also kills plants abruptly. Careless use of a lawn spreader, spray drift, and on a hot day possible volatilization of an herbicide spray can cause it to affect unintended plants.
Slug damage on hostas is fairly easy to recognize as they eat large continuous holes in the foliage and leave trails of "slug slime" behind on the plants an on the ground. They work at night, so you won't see them, but you will see more damage every day. In my experience they do not bother most other shade plants, so I would be surprised if that was what killed your plants.
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