Drought - you don't say where you are, so I don't know what your weather's been like, but either way, you need to give it a few litres every few days. If you're not convinced, gently dig it up and inspect the rootball - bet its really dry. Water now either by leaving a hose trickling at the base for an hour or so, or applying two watering cans full of water around the base, so it gets between 5 and 10 litres of water immediately. Ongoing, you need to give it enough so the water can penetrate down to 6 inches or so, which is where the root ball goes down to, rather than a light watering every day or less often. Unfortunately, the degree of leaf wilt may mean they don't recover and it drops those leaves, but keeping it watered ongoing will at least save the plant.
This plant is in full flower and its only recently been planted - that means it has a greater requirement for water because of the flowers, and it has not yet had a chance to put out its own roots to seek water for itself. Keep it well watered up till autumn.