Hi Martin, I think this happens to older plants. The rhizome also stores food for the plant to grow.I think the problem arises due to our cold damp winters,the plant needs the food stored over winter to get going again in spring,we all grow sarras crammed in ,side by side and not all the plants can get a good feed of insects to replace the energy used to grow all the pitchers over a full year,this has the effect of draining the rhizome of its stored starchy food reserves over time,so it dies and the rot spreads down the rhizome and gets worse due to our cold/damp winters. that's my view anyway.