What was the health of the roots prior to repotting? Were they actively growing? If not, repotting tends to be harder on the plant and existing roots. Were any bad roots trimmed off when the plants were repotted? What are your growing conditions? Temperature? Humidity? Air circulation?
The bottom line is when plants rot, they are staying too wet for too long. This is related to watering frequency, temperature, air circulation, and humidity. The cause can be one or more of these factors as well as the general health of the plant. If you have multiple plants affected that were previously healthy, then the problem is the culture/conditions. Another thing I do with freshly repotted plants is to keep them on the dry side for up to a month. Repotting tends to make the plants more susceptible to rot/disease due to stress and possibly broken/cracked roots.