I've had a similar issue with my summer bearing raspberries from Nourse. Mine were planted on a new hugelkultur berm that was made from cleared brush and subsoil from digging a pond. They started off great, got a few feet high, and did the exact same thing as yours.
I talked to the owner of Nourse farms (I sent an email to them asking for guidance and was pleasantly surprised when he called me) and the issue with mine seems to be lack of organically available nutrients they needed to establish themselves fully. They basically expended what was stored in the bare roots and burned themselves out. Once they were in a weekend state they started to be attacked. Rather than compost, nourse recommended trenching the sides and generously applying fertilizer. I did this and managed to save about half of them.
I don't think over watering is an issue. I have another set growing on a year old hugel that is directly bordering an irrigation ditch, and the soil is continually damp. Those are producing like crazy, though I'm snipping off the berries when they form to let the plants get well established.