The leaves may have suffered some fertilizer burn from the fertilizer or they could be dying back because of the excessive moisture. I would remove some of the dead and dying leaves and try not to keep the soil overly moist when watering. You may want to repot the plant at some time to put in some rough compost, shredded bark or something like that to increase drainage if the soil stays wet all the time. You want the soil moist but not sodden wet. If either of these problems are causing the leaf die back they should be temporary. The fertilizer should eventually wash out and if you let the soil dry some the leaves may gain a better condition. If neither of these work you may wish to consult with your local county Ag extension agent regarding problems with raspberries.