New variety. Co-incidentally I was talking about this particular plant with a supplier just two days ago. He was saying that it's pretty tough for a large flowered tree. No particular needs re soil type, just not shallow soil over chalk, or too acidic. Sheltered site, which will protect it from drying winds, and a well-drained soil. All I can think is that perhaps the sudden hot spell plus the breeze have stressed it a bit.
Check the soil around the base to see how damp it is. A plant's first reaction to drought-stress is to shed what it doesn't need so that it protects its core from further water loss, so the leaves go flabby. How long has it been planted? Is it possible that the roots haven't made growth yet, so they're still all in the pot-shaped ball of compost and have dried faster than you thought?