It's that thing that irises and daylillies are also subject to - too many of the same plant in a given space and they tend to die out. It's not a disease thing, though that can have some effect, too. The container fills up with roots and stems, and then there's nothing left to grow with. I don't know that raspberries would fall victim to this tendency, but extrapolation on their growth patterns leads me to the speculation, if you like.