nothing is the short answer, that is their growth habit.
check their pruning group and prune accordingly. you could bend some of the new whippy growth down to disgusie the stems or grow a small shrub or herbaceaous perennial infront of them.

If your clematis are viticella group late flowering ones (starting around late June and going on till September/October), the way to get round this is to hard prune them during winter, but not later than mid February, right down to about 2 buds from the ground.

Yes, I cut back my viticella much later because I tend to let the insects have their hibernation time. All the dead brown stuff was cut back eventually and it sulked for a long time. It's going alright now.

Thankyou all