These aren't growing outdoors? Otherwise, I was going to say it might be because of winter, since Opuntia will drain their pads of water. Mine (planted outside) are a little brown in spots, but I'm fairly certain this will clear up when spring is in full force.The other opuntias in the same pot seem fine. I don't think it's to much water, it's in a well draining soil and I don't water until the soil feels dry at about 3in depth.
Plan on potting in separate pots later this spring but now I'm considering doing it sooner in case it's some kind of disease.