Strange indeed!
Any chance that you had a heavy, cold spring rain in March after your ponds already started warming up? That might have caused a rapid turnover of the water and a low oxygen event. If the CC were mostly large fish, it might have killed most of them before the BG.
Also, are your ponds getting close to their carrying capacity of fish mass? CC are very efficient at growing large in Texas ponds and BG are very efficient at creating large numbers of BG. If those fish are not culled periodically or controlled through predation, then your ponds could just be packed with fish. In that scenario, any stressful event can be magnified in how many fish it wipes out.
Finally, any chance you have otters in the area? One family that found a pond full of tasty catfish, could easily clean them all out over a single winter season. They would then probably move over to the next closest pond, and repeat their fish slaughtering process.
