It sounds like you have provided the plants good care, and that they are in a good setting for them to stay strong. I suspect if any thing is wrong the extra fertilizing and perhaps a little to soggy of soil conditions are at fault, but this is just a best guess. You might try repotting them and cut away any rotting roots in the process. Then replant in fresh moist soil and don't water until the soil is slightly dry to the touch an inch or two deep.
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