The grass behind looks rather droughty. When you bought the plants were they established plants in soilless media in a pot, or were they plants that were dug up for a plant sale or something like that? If the first, was the pot watered well before it was planted? As Elaine said, the leaves are scorched on the edges, which could mean too much fertilizer. It might also mean drought. That can happen even if you water a lot after planting if a root ball of soilless media has dried out significantly. If you lift it as Elaine suggested you would be able to see if the media is overly dry. If it is, soak it in a tub of water before replanting. It sounds as though it had only been in the ground a couple of weeks before it started its decline?
As far as Liatris is concerned, I had some wiped out this year by voles chomping on the roots.
