Since it's only the bottom row I'd look more towards a fish nipping at it or something close by, other LPS sweepers stinging it (sweepers can get very long), growth from the rest of the colony shading the lower heads, power head moved, the xenia maybe hitting it causing the heads to stay closed?
A lot of the time softies will die back when nutrient levels are to low.. I had trouble growing softies with my predominantly sps tank.. Try some aminos... But first make sure that carbon was rinsed well..