Most likely some of the stems got damaged by some creatures like slugs. When slugs scrub clematis stems some of them show damage right away, some show it later when temperatures hotter. Look at your plants. Is it the whole stem gets dry. If this is the case cut it and don't worry. New stems will emerge. If it's just lower leaves get brown that's pretty normal too. Just don't overwater or overfertilize . Even if (in the most unfortunate case ) the whole plant would succumb there's 90% chance of getting back in spring. They are tough and want to live and grow.