I obviously can't tell for sure what's the problem, but you should keep in mind that your bluegill population may have a natural annual mortality up in the 30% range. If you really think about how many bluegill you have in your pond, it would be highly likely that you'll see mortalities every spring as the fish come out of the stressful winter period. I wouldn't be terribly worried about it.

Holding a redear sunfish is like running with scissors.