Does the growth at the top eventually die out? If it does I would guess they are rose cane borers which deposit their eggs in the canes. Look around and see if you can find a dead spot in the cane that continues up from that point. Everything beneath that should be living. One has to cut the cane off beneath that point and dispose of it so the pupae can't continue to develop and drop in soil. If it's not that I don't know what it could be. Perhaps cross post this into the rose forum.

Otherwise it could be the very moist soil catching up with them. Another thing I am aware of is delayed failure of scions of named forms of Rosa rugosa placed on incompatible root-stocks, after the finished plants have been sent out to customers (don't ask how I found out about this). But if your specimens are not grafted this certainly won't apply.